What causes homosexuality?

K, I'm not a homophobe. Seems like the gay marriage issue has been in the news a lot more in the past few years. I have a strict policy with myself that I do not take a stand on any political or social issue until I feel like I understand it. I won't accept the Lady Gaga answer of "I was born this way", if homosexuals are born this way, why? Maybe some are born with it, and maybe others develop it later in life, I don't know, I'm completely in the blank on the issue. Honestly, I doubt there is anybody who really understands what it is. The thing that confuses me about homosexuality is its place in the world from an evolutionary perspective. I know homosexuality has been observed in several species, but if a species adapts over time to increase its likelihood of reproducing (some of those adaptations being quite subtle) why would so many individuals outright reject reproduction altogether, and mate with the same sex? Isn't that counterproductive to the biological purpose of life? I'm not trying to argue against homosexuality, I'm just writing down my thoughts and hoping someone can enlighten me. I know it is frowned upon to suggest that homosexuality could be a disorder, but if it is natural, as the gay rights groups suggest, what is its evolutionary role?