The reference in the article to Sullivan's bragging about his glutes suggests that he is the bottom in this circumstance. It is possible to contract HIV by being the bareback top--especially if one is, as they say, well endowed (ie. likely to tear the muscles of the rectal wall). But the odds are much less than being the bottom. (Even if you have unsafe anal sex w/ a positive partner [ie you're the bottom], you are still only talking about 1% chance of infection.)
I hope that Sullivan gets savaged--not for his sexuality, but for his hypocrisy. If you want to put sexual practice in your political agenda, you can't expect people to feel sympathy for you if you are basically acting out your own sexual arrogance--cf Sullivan's "When Plagues End" a few years back in the NYT magazine.
Barebacking itself is a predictable result of public health disinformation about HIV transmission and risk, (which isn't to say that accurate information would be the best thing for public health, measured by transmission rates), as it foregrounds the ethical (not just epidemiological) choices involved in sexual contact. It has already spawned subcultures and pornography which advertise themselves in the register of "men doing what men do to each other, without fanfare or bullshit" etc, and reference to pre-AIDS sexual practices. (70's porno is great for both this and its awesome soundtrack--incipient disco.) But its incipient nostalgia isn't sentimental--it hypermasculinizes risky behavior, without longing for or mourning the post.
Christian