The incidence of unprotected sex is skyrocketing in America among younger people. I don't have the stats at hand but my partner is a public health scientist whose job is to measure behavior and design intervention studies in young populations. (He is also a guinea pig in a vaccine study.)
These people are not uneducated. They know perfectly well that unprotected sex can get them infected. Personally, I don't think you can explain the increase by use of drugs in recreational sex or by the idea that HIV is now a treatable disorder.
I think it relates more to the "official" attitude toward sex promoted by gay media, which has become quite conservative and sex-negative, promoting monogamy, gay marriage,e tc., as the highest virtues of gay life. This in turn leaves younger gay men with a sense that they can't talk openly about their "promiscuous" behavior and, consequently, they don't learn how to negotiate themselves as effectively in sexual situations. I've seen this repeatedly with clients.
The grandest example of this was the case of HIV-positive Andrew Sullivan last year. After making his name with the book "VIrtually Normal," an argument for the value of marriage and monogamy, he turned out to be advertising for unprotected sex all over the internet.