His name was Derek*, and he was a military pilot.

We met in a noisy bar in the Castro a few years ago, when I was passing through the city. It was an unremarkable night, as predictable as the Christina Aguilera videos playing or the Bass I always ordered. He told me about his exploits, including the time he almost got shot down by friendly fire on a mission. I'm not sure I believed him, but I didn't really care. He was close enough to my type: shorter, with a deep baritone voice and a dairy-fed look. We went back to his place. I'd probably have forgotten all about it, lost in a blur of one-night stands, if it weren't for one thing.

It was the first time I had unsafe sex.

I hadn't planned on it. But after several drinks and some fumbling with a comdom that left a part of my anatomy, and my ego, a little deflated, it just happened. Rather than risk condom failure again, we took a different risk. The whole time, I felt both ecstatic and nauseated. Was I contracting HIV at that very moment? Eleven years of vigilance about safe sex was suddenly coming to a nerve-racking climax. Sensing my anxiety, he mumbled, "I'm in the military—they test us every six months."

For days, though, all I could do was worry. When I talked with friends about it, they excoriated me for being self-destructive. Some used the term "death wish." What scared me most was the nagging feeling that I could no longer trust myself to be 100 percent safe. The world didn't end, however, and panic slowly receded into complacency. An HIV test confirmed I was negative. Then, about a year later, I slipped again. The temptation to "go bare" was everywhere, and the guys I met seemed lax about my wearing a condom. It was hard to resist. Sometimes it happened with casual hook-ups, other times with people I was dating.

With each slipup, rationalizations filled my mind. As a top (usually), my chances of catching the bug were slimmer, right? (Tell that to Magic Johnson.) Wasn't it mostly crystal meth types who were getting infected, not those clean-cut, professional guys I was hooking up with? A new pattern seemed to be emerging for me. Have sex now, rationalize later. I confessed to friends once again, hoping they'd knock some sense into me. This time, their reactions weren't what I had expected.

"It happens," said one friend, letting me off with a wag of the finger. As it turned out, some of our friends were also slipping.

Six months ago, the Chronicle ran a front-page story about the Centers for Disease Control's (CDC's) latest HIV figures. After years of having one of the largest numbers of infections per capita, San Francisco was one of three metropolitan areas in the country to experience a drop in that figure. From a 2.2 percent rate of infection among people tested, the figure dived to 1.2 percent. The prevalence rate—the number of people living with HIV—dropped from one in three gay men to one in four.

The results were unexcpected, to say the least. Only five years ago, in another Chronicle piece, Dr. Willi McFarland, director of HIV-AIDS surveillance for the S.F. Department of Public Health, had compared the city's HIV transmission levels to those in Africa. There was mounting evidence that gay men were using condoms less and less, and those under 30 were purported to be increasingly promiscuous. HIV looked set to claim a new generation.

Only doomsday never happened. Health officials and HIV experts found themselves scrambling to explain the good news. "What this shows is that the vast majority of gay men are succesfully taking care of themselves in preventing the transmission of HIV," says Dr. Steven Tierney, director of HIV prevention for the S.F. Department of Public Health. There were nods to the city's prevention efforts, including regular testing and campaigns aimed at curbing the use of crystal meth, a drug that lowers inhibitions. (Users are three times as likely to contract HIV as nonusers.)

But one factor in particular attracted attention as an explanation for the drop: positive men in the city had developed a culture of having sex with partners with the same status. This trend, called sero-sorting, was brought to wider attention in the medical community by doctors J. Jeff McConnell and Robert Grant of the Gladstone Institute of Virology and Immunology/UCSF. Their ongoing study, Positive Partners, has tracked over 175 positive men, who racked up more than 5,000 episodes of intercourse in the three-month period prior to their interviews. What the doctors found was startling.

The majority of positive men—60 percent—were limiting their riskiest sexual activity (condomless anal sex) to others who were positive. Those who had sex with negative men or men of unknown status mostly used condoms. When they didn't they took on the role of receptive partner to minimize the risk of HIV transmission. The investigators had estimated there would be 116 new infections; that figure dropped to 6 when sero-sorting was figured in. "People were clearly worried about spreading the infection to others," says Dr. Grant, "and either developed skills or preferences to have sex with others of the same status."

One reason for such responsible behavior may be our gay community's unique level of awareness about the disease. The June 2005 CDC study found that San Francisco had the highest percentage of testers (77 percent know their status, versus 48 percent in New York) and the lowest percentage of HIV-positive people walking around thinking they're negative, at only 23 percent. Contrast that with 42 percent for Los Angeles, 52 percent for New York, and 62 percent for Baltimore.

Then there's the relative openness here about HIV status. When I moved back from New York in 2002, men seemed more comfortable asking each other about their status. Everywhere else I'd been, the culture seemed to be "Don't ask, don't tell." On the Internet, things were even more direct. From craigslist to Bigmuscle.com, HIV status was rattled off along with favorite books and music or sexual positions. Fifteen years ago, at the height of the epidemic, the city had been in a state of viral aparteid. These days, seeking out someone of your own status raises as many eyebrows as a drag queen adjusting herself at Trannyshack.

It wasn't long before experts seized upon sero-sorting as a prevention strategy. Apart from the problem of STDs, the only downside for positive men was the potential threat of superinfection—infection with a new or second strain of HIV. Except in cases of HIV contracted in the last couple of years, experts such as Grant and McConnell and Dr. Jeffrey Klausner, who oversees STD prevention at the S.F. Department of Public Health, have largely ruled out superinfection, based on studies. But as the prevention community celebrated and debated the merits of sero-sorting for positive men, there was a wild card that experts hand't publicly addressed. Was it possible that negative men could successfully sero-sort, too?

When I first read about sero-sorting a year ago in the gay press, I thought, Aha! Here's my HIV hall pass. If the majority of pos guys were having sex with other pos guys, and I was meeting mostly negative guys, the odds were in my favor, right? While I didn't sprint to the nearest bar for a spree of unsafe sex, I felt a rush of validation. The anxiety I'd had over slipping was assuaged. But only somewhat.

Negatuve guys like me constitute the bulk of the city's gay population—75 percent. We've supposedly been the "good" kids in that rainbow-colored dysfunctional family known as the gay community, and the attention has gone to the "problem" kids. Was it assumed we were practicing what we'd been taught for decades, that condomless sex equals death? "Most of the media attention is on nonwhites [gay African American men have the highest infection rates] and subcultures like the crystal meth scene," says my friend Chris Panizzon, 31, a Marin native who lives in L.A.

Still, behind the scenes, negative guys are increasingly engaging in a form of sero-sorting. "It's based on what I hear anecdotally, but I do think the condom code has gone away," admits Jeff Sheehy, a prominent community activist and the mayor's adviser on HIV and AIDS. "Community mores have changed."

In the past couple of years, studies have confirmed what the experts have suspected for some time and what the community's been chatting about at dinner parties or the Bar on Castro. The New Yorker, in a 2005 article discussing the role of Internet hookups and crystal meth in the spread of HIV, cited a study that said that for the first time since the safer sex campaigns of the eighties and nineties, the number of gay men reporting that they regularly used condoms had dipped below 50 percent. And this summer San Francisco Gay Men's Community Initiative, a local activist group, released the results of an online survey asking 500 gay men in the city about their attitudes toward sex, among other things. Obviously, an online survey doesn't carry the same weight as a scientific study, but the results are interesting: almost 20 percent of negative men said they now felt that condomless sex was the community norm.

Adam Simmons, 23, originally from Georgia, says he always uses condoms but is in the minority. "I can't think of one time [back home] when I heard someone say, "Oh, don't worry about using a condom, just this once.' Here, it happens all the time."

For us negative guys, the complications of sero-sorting are obvious: we can't ever know a casual sex partner's status with certainty. People might not know they've become infected since their negative result; worse, some could lie. So, even if we ask straight out and are told "I'm negative," we have to make our own judgement about our partner's HIV status. This is filtered by a complicated matrix of variables—some more reliable that others—ranging from the stress of a lousy workday to how attractive the guy is (after three or four cosmos, everyone's Brad Pitt) to how much we trust him. Whether we're topping or bottoming (the latter is about ten times riskier) also figures largely in our decision to "slip."

In my night with Derek, all those factors came into play. At the risk of sounding like a Pollyanna with circular logic, Derek just didn't seem the wild type and sounded convinving when he said he was negative. And it did cross my mind that since he was a passive partner, he was bearing most of the risk.

My friends, it turns out, have also succumbed to the temptation to go "raw"—and to believe that they can suss out someone's status. Aaron Christianson, 39, an IT professional, just ended a period of slipping because he discovered his boyfriend of two months was positive, not negative, as he assumed. Over his ten years here, he's noticed men becoming less concerned about using condoms, which has made it more difficult for him to resist. "There's a tremendous amount of people out there who are willing to have sex without condoms," he says. "If you're only screwing within your circles, it seems a reasonable assumption that you'll be OK. That's how I let my guard down, because there was this feeling that I was sero-sorting."

In some ways, it's surprising that it took so long for safety standards to loosen. The safer sex campaigns were developed in the eighties as an emergency measure to stem the epidemic. People hoped, perhaps unrealistically, that a cure or vaccine would be developed in a matter of time. Gay men being men, full of testosterone and, like most straight men, loathing condoms, it was unrealistic to think everyone could maintain 100 percent vigilance indefinitely. Condom fatiguw eventually led to the glamorization of skin-on-skin contact. By the midnineties, an entire genre of gay pornography—barebacking films—had sprung up to tap into gay men's fantasies of condomless sex. As for me, I always wondered how long I could keep my guard up, resenting the fact that I'd missed the seventies sexfest but thankful that I was at least alive. Still, I sometimes asked myself, how is it that as a homegrown gay man, I have only one HIV-positive firend, and he's a survivor from the eighties?

What really transformed the safer sex culture, though, was drug cocktails. When they hit the market in the midnineties, AIDS morphed from a harrowing death sentence into an often manageable chronic illness. Perpetuation the new optimism, ads from pharmaceutical companies selling the cocktails depicted studly guys rock climbing and biking, which made it seem as if AIDS was little more than an inconvenience that could be controlled with a pill a day, like taking a multivitamin. All the same, enough people were concerned about the advertisements' messages that they were eventually pulled. "What the companies did was criminal, says Ralph*, 41, an entrepreneur. "It totally encourages complacency." But perhaps the ads merely reflected the complacency brought on by the steep drop in AIDS-related deaths. A new generation of gay men had come of age, largely insulated no only from the horrors of AIDS but from the disease itself.

Naturally, sex without condoms remains a risk. But to what extent? The official line, of course, has to err on the side of caution—prevention experts can't exactly advocate selective safer sex. "Sero-sorting can be a flawed strategy for negative men," says Jason Riggs, the spokesman for the Stop AIDS Project, based in San Francisco. "They can be putting themselves at risk."

Still, different people encounter different levels of risk, and most everyone has a vague idea of where they stand. Someone who picks up several guys a week and does drugs regularly is tempting fate more than a twentysomething who dates friends of friends. (Stop AIDS says about 30 percent of new HIV infections are linked to crystal meth usage within six months prior to testing.)

"It may sound judmental and that I'm going off a certain stereotype, but I do think the circuit-party crowd has higher drug use and is riskier," says Simmons. He thinks older men make up another risky group.

Statistically speaking, he's right. "If you look at the prevalence of HIV in San Francisco, the number is one in four," says Sheehy, the mayor's HIV-AIDS adviser. "But among the 35- to 55-year-olds, the rate rises to 40 percent, with rates dropping off markedly above and below those ages." This is despite the fact that in CDC study of 15- to 20-year-old gay boys and men nationwide, a whopping 41 percent reported having unprotected anal sex. Yet a new epidemic didn't strike this group, because they were mostly negative to begin with. Moreover, many positive guys who were with negatuve guys had such low viral loads that they weren't highly infectious. All of which begged the question: was the HIV risk overstated for some groups?

Stop AIDS is currently trying to answer that question. As the largest collector in the world of data on gay men's sexual practices (gathered from about 10,000 men year-round), the group has kept tabs on the local community for two decades. Through its own data, that of the CDC, and that of the city's clinics, the team tracks the virus like private detectives do, talking to sources and sifting through studies to determine how and where the invisible enemy moves through the community.

Last July, Stop AIDS adopted a radical prevention strategy. "We're clearly seeing a shift in the gay community and the epidemic, and that's why we've retooled," Riggs says. The group borrowed a technique from the social sciences, calling it sexual network theory. The approach maps out the community much like an electrical grid, with HIV striking certain segments disproportionately. The goal is to help prevention outfits better evaluate each client's risks and educate him about them.

Stop AIDS found, for instance, that Asian Americans, white Americans, and African Americans tended to have the same types of unsafe sex with about the same frequency. But their HIV rates varied dramatically. Among gays, Asian American men reported the lowest HIV rates, African American men the highest, whites somewhere in the middle. Rather than focus solely on behavior, the theory holds that what drives an individual's risk is his position within the city's sexual networks. "It's really the sero-prevalence within a network and how it's structured, says Riggs. "It helps us contextualize how likely someone is to get HIV."

Think of the Castro as one big Harvey Milk Gay HIgh, with a multitude of cliques. The leather boys might be hanging out on the lawn smoking cigarettes, the preppy guys mostly congregating at water polo practice, and just about everybody passing through the cafeteria or gym class at one point. These cliques are aware of each other, but they don't often overlap. So even if one group ends up passing, say, mono among its members, other cliques won't necessarily get it.

There's a catch, though: stay "connectors," people who move between groups. Therein lies a key weakness of sero-sorting for negative men. "THose who are low risk can quickly become high risk if the sexual network gets infected," explains Riggs. In other words, that seemingly prudent, mild-mannered young Republican you assumed was negative might secretly be hitting the sex clubs and end up infecting his circle of low-risk friends.

Even with its flaws, the theory reflects a more realistic view of what the gay community has become. Thirty years ago, everyone from doctors and CEOs to plumbers and policemen slept their way through the bars and the baths. Being gay (and discriminated against) was an equalizer, the sex wasn't negotiated by psychographics nearly as much as it is today. Nowadays, there are bears (heavy, hirsute men) who only "woof" at other bears. Others sniff that they don't "do the Castro," while still others stick to house parties or cruise the gyms.

"In the past, we focused mostly on high-risk people," says Riggs. "Now we can focus on people who move between high and medium risk, 'bridges' who are slipping between groups. From there we can do structural intervention [such as distributing condoms at a sex club] to modify the structures of those networks."

In today's kaleidoscopic culture, the dialogue about HIV has to be nuanced. "The cut-and-dried approach doesn't work anymore," concedes the health department's Tierney. "Our job is to respond to the realities of people's lives and try to increase their safety. There are some situations where you definitely use a condom, and there are some when you don't." Riggs adds, however, that "slipping is not necessarily a conscious decision. It's contextual; it's an accident. It could happen for many reasons. That's the conversation that we need to have right now."

But how ready are gay men for that conversation about what "safe" means? The prevention community and the media haven't exactly had a perfect record in predicting risk levels or AIDS trends. The conflicting, sometimes amorphous information has bred confusion. So the old messages stick with us; forgetting them is as easy as unlearning how to ride a bicycle. "There's guilt associated with [slipping]," says Panizzon, who mostly plays it safe. "But I also think of it as taboo and get a rush out of it."

Another friend, Sam*, 40, a successful attorney, can relate. Although he's taken a few "calculated" risks, he doubts he'll ever feel comfortable about condomless sex. "It's how I grew up; it's in my makeup now," he says. He tells me about the time he was on a second date with a guy in Puerto Vallarta. "Before we had sex, we talked about both being negative," he says. "I took an educated risk. But not long afterward, I had the flu and night sweats. I freaked out, called my best firend, and panicked out loud about whether I was sero-converting [the body's reaction to having contracted HIV]."

Ralph, the entrepreneur, defends the old safer sex standard. "I came out in '84 at the height of the fear and the crisis," he says. "I've adapted my sexual behavior and generally don't have anal sex." I try debating with him, bringing up the drop in infection and prevalence rates, the concept that the majority of positive guys here are responsible, have low viral loads, and aren't hugely infectious. Still, no dice. "A negative guy would be an idiot to think that he could sero-sort," he replies. "It's a terrible mistake, and you'd end up with a lot of sero-conversions."

Certainly, you'd expect younger gay men, who haven't witnessed the worst years of the disease and who don't have many friends with HIV, to be more willing to leave the condoms in their nighstands. "I'm surrounded in my professional life by men who are in their forties, and they view HIV as an epidemic that killed most of their generation," says Bayley Riaz, 26, an administrator at substance-abuse treatment center Walden House, who says he has "sero-sorted in the past."

"It's not as scary a proposition for people in my generation," he continues. "Maybe I'm in denial. But for me, it's a realistic thing. To live in constant fear is worse than not living at all." Riaz tells me that while he's mostly safe, he has even topped guys he knows to be positive without using a condom. "Maybe I just like to take risks," he says.

Younger gays aren't the only ones hoping to play by a new set of rules, though. A friend of a friend, James*, 41, a doctor with a significant gay practice, has been thinking about these issues for some time now. "It's so fatiguing to have safe sex at all times," he says. "I see most people stray from the condom code—but with guilt and without knowledge. I seriously wonder if sero-sorting could be a viable option for negative guys, what the methods are, and how much testing is required to make it work." He advocates making a rapid HIV test available to the public. (At press time, the FDA was considering allowing this to happen.) "It coupld revolutionize safer sex practices. I've had conversations with guys online who say they're negative, and I've asked them if they would take an HIV test at my house, and the conversation changes. I've used that as a safety gauge."

Sheehy shares the good doctor's sentiments. "I believe that a rapid HIV test should be sold over the counter and that people should be regularly tested," he says. "We will always need condoms. But at the same time, sero-sorting in the entire community could be a stategy for eliminating HIV. You've got the cocktails and the prevention potential for treatment. You've got sero-sorting and widespread testing. All of this can help."

As I listened to all the conflicting opinions about whether condomless sex can ever make sense for negative guys, I found myself equally conflicted. Part of me wondered, well, why can't gays finally live the way some of my straight friends do? Am I that much more at risk than they are? A more nuanced approach gives people hope and eases the guilt that makes them do crazy, dangerous things. I thought back to my coming-out days in the early nineties, when even oral sex seemed risky. Every time I got a bad cold, I'd wonder if it was the Lewinsky I'd gotten a few weeks before. Sure, everyone did it without condoms, but it took a while before experts confirmed what people know anecdoctally: oral sex is very low risk. Could negative sero-sorting be on that trajectory?

For me, however, even if I slip again, emotions always trump logic and science. Now in my midthirties, I can't forget my wonder years here in the city, when I saw people walking around with crimson-purple lesions and gay activists from the Castro visited my high school in 1984 to tell us that we couldn't catch AIDS from the 24 Divisadero. I was the safe sex generation. I even worry that this article might influence just one person to take a chance. And, God forbid, to sero-convert.

Last year, bored with the environment at my gym, I decided to switch to one in SoMa. It was like working out in a leather bar with lots of grizzly, pumped-up guys in their forties, but there was nothing more motivating than to see everyone benching 225 without breaking a sweat. (Maybe those were just the ones I noticed.)

Two weeks into my new routine, amid the equipment and the men and the blare of house music, I spotted Derek. His hair had thinned, and he'd put on some weight—the MO for many guys once they partner up. I recognized the guy trailing behind him; he'd been Derek's friend before, and now they were clearly together. When Derek and I walked by each other, we grinned our hellos. It was both friendly and awkward. Still, it was reassuring that four years later, he looked fine, and settled. I had made many assumptions about him the night we met. It seemed as if I'd been right.

As a writer, I'm paid to trust my instincts and make judgments about people, sometimes based on just a couple of interviews. Can I rely on those instincts as a single gay man? I take risks with everything else in my life. I thought back to the moment when Dr. Tierney punctured the formality of our interview and let the camp, fiftysomething single gay side of him poke through. About these risks we take in life, he cautioned "The one sure way not to get hit by a car is to not cross the street." Then he added, "But it makes it hard to get to the Midnight Sun." Perhaps. Hopefully, thought, my bachelor days are numbered. Better to let someone else have thant conversation about slipping.

* Names and some identifying details have been changed.