Interesting question, junebug. It's come up here before; a search of the "Sexuality, Reproduction" board will find you some past discussions if you're interested.
I'm just a layman, but one plausible reason for the conservation of homosexuality is that it's possible to contribute to the perpetuation of your genetic line even without reproducing yourself. I'm a hetero male, for various reasons unlikely to have kids of my own. Yet I can and do help my sister with looking after her son, my nephew. Aside from the emotional connections, I'm clearly benefiting from aiding his development.
In any earlier era of human history, such contributions were even more crucial. Homosexual men and women could still hunt, gather, craft, stand watch over their kin, just get up and tend the babies in the middle of the night while others slept. Indeed, they still do these things.
I agree with One Above that overt homosexual behavior (actually preferring the same sex) is a combination of genetics and environment, the precise details of which aren't known yet. Clearly it doesn't happen often, but that it does happen at all shows the genetic component is widespread and therefore has yet to be selected out of the human genome.
To be clear, this is uninformed speculation, but it seems to me this means homosexuality can be seen as a neutral mutation, neither offering great benefit (or we'd see even more of it), or great disadvantage (or it would've already been selected out). There are others here with a far better grasp of genetics than I; perhaps one of them could correct me.