There is a notion that a number of people have that finding the biological basis of homosexuality ("gay gene" or fetal hormone levels or a host of other possibilities) is the key to civil rights and social acceptance for gays. The thinking goes that if want we can say to homophobes, "See, we're born this way, so it's beyond our control! You can stop hating us now and let us get married!" But I think that is just a totally naive notion. Black people are born that way and have no control over their skin pigmentation, but plenty of people are still racists; so too would there be homophobes even if science proved homosexuality was innate. Bigotry is a stubborn, nasty thing, tenaciously clinging to the human psyche with tentacles of irrationality, fear, and hatred even in the face of empirical evidence or even simple reason. Those who already hate gay people will continue to do so. We'll just see some stunning hypocrisy about genetic engineering "to save our children!" And I can hear cries of, "People are born schizophrenic, but we still treat them!"
There is something a bit unsettling and self-hating about this notion of acceptance-via-biology, too, though I don't think a lot of people who hold this view realize it. It's like, "Yeah, it's dirty and shameful, but I was born with this affliction! Of course I'd rather be straight and normal, but I can't control my biology, so get off my case!" Not really a positive affirmation of self-acceptance, I'd say.
It doesn't matter if we're born gay or "choose" it or it's all our mothers' fault for putting lipstick on us that one time. We deserve civil rights such as marriage because we're human and we're citizens, because our love is just as real, our feelings just as valid, as anyone else's. We should be able to love who we want for whatever reason we want (within the bounds of sanity and consensuality), because we're free human beings. American history, is one long slog to live up to our extraordinary ideals, to bring what is universal, but abstract, into the realm of the particular, and actual. Those who deny the rights of gays or transgendered or any minority are on the losing side of history, betrayers of our national soul, small, frightened minds rejecting the human feeling and understanding that makes us worthwhile as a species and representing the worst impulses that make us damnable as a species.
BTW, personally, I do believe homosexuality is innate, but I think from what we can tell so far, there are a whole complex of causes, not just one "magic bullet."