Echoing what others have posted, I think gay guys have a much easier time getting casual sex -- since there are no women involved, and women tend to expect something in return for sex, things like commitment, attention, monogamy, or (at the highest and lowest levels) money.
For guys, getting their rocks off with no strings attached is often enough by itself. For the same reason, I think it's harder for gay guys to sustain a serious relationship because of all the tempation coming from all sides and guys' own roving eyes.
It's been pointed out that in recent decades young straight women are more willing to hook up for its own sake, so (at least for the younger generation) straight sex is growing to resemble gay sex more and more.