Over at Plasticbag, Tom’s got a pretty interesting discussion going on about genetic influences on sexuality. He makes a point of noting that he’s not an evolutionary biologist, but he’s clearly got a very good grasp on the most important point. The issue is that no-one (in the discussion at least; possibly no-one at all) can come up with a convincing advantage that might accompany homosexuality that would balance the fact that, clearly, gay people are less likely to reproduce.
The best suggestion so far, I think (and I would, because it’s mine), is that exclusive homosexuality is a relatively new phenomenon and one that only seems to exist in humans, so it’s likely to be a phycological or sociological extension of the more balanced bisexuality seen in many vertebrates.
The actual post includes extensive quotes from a New Scientist article (not available online) which has a very compelling explanation for why we can assume homosexuality has some evolutionary advantage, or at least no net disadvantage.