What Gustaf said, plus I think the issue here is dudes more than gay dudes.
I think the recipe for gay male promiscuity is pretty simple: Women are very unlikely to be highly promiscuous; men are much likelier. So, a very disproportionate amount of highly promiscuous sexual encounters are going to be among gay men.
I also think there are some other issues here -- e.g., gay men are probably a bit more likely to reject societal norms because they feel society has rejected them, or are more likely to try to annonymize their sex life -- but I think it's mostly the dude/dude issue.
This.
It's still repulsive, of course, and AIDS is a menace to society because of behavior like this.
