PZ Meyers links to some possible explanations for the evolution of homosexual behavior. Though he's right that a lot of the reasons on the list can be collapsed into adaptionist (it somehow helps genes propagate) and non-adaptionist (it doesn't help genes propagate, but it sticks around anyway) reasons.

I first heard the kin selection argument as an undergrad, and I thought it was fairly convincing at the time, though now I'm more skeptical. The idea is that genes that induce people to like the same sex keep them from actually reproducing, but lead to behavior that enhances the chances of their close relatives' genes doing well. This is kind of a "worker bee" theory, where there are non-reproducing neuter drone bees whose job it is to help the propagation of those bees who can reproduce. This idea relies on the assertion that the behavior of homosexuals is actually beneficial to their kin, but that's a very difficult claim to actually verify.

The other class of explanations basically say it doesn't help genes propagate at all. It's either vestigial (like the appendix or male nipples) or possibly deleterious. But it just keeps hanging around because there's not enough evolutionary pressure to eradicate it. I'm beginning to lean more toward this point of view, based on a more nurture-centric view of mind. If love is somewhat analogous to imprinting (e.g. when a baby duck imprints on the first moving duck-like thing it sees that might resemble a mother), then attraction might be a result of latching on to particular features that are desirable. If those features are in a prospective mate who happens to be the same sex, then so be it.

There's also the fact that men, in the absence of females, will revert to homosexual behavior. Increased homosexual behavior in prisons and on the battlefield are well-documented.

I don't like the "homosexuality gene" explanations very much because they seem too simplistic and black-and-white, while homosexual behavior seems to span a continuum (there are, after all, bisexuals and a spectrum of behavior in between the polar opposites).

Also, anecdotally, it seems to me that the small number of homosexual men that I've known have stronger sex drives on average than heterosexual men. I'd be interested in seeing specific studies linking sex drive and orientation.

EDIT: Hah, I just ran across this:

Recent research suggests that, for most women, high sex drive is associated with increased sexual attraction to both women and men. For men, however, high sex drive is associated with increased attraction to one sex or the other, but not to both, depending on men's sexual orientation

Anyway, I think the phenomenon is still poorly understood, and because it is highly-charged and controversial, that makes objective study of it even more difficult. But it's definitely a fascinating subject.