I actually don't care if it turns out that homosexuality is genetic. It's irrelevant to the moral question. Just because homosexuality may come naturally for some doesn't mean it's moral to practice it. Indeed, one could say that the difference between just doing what comes naturally and principled self-restraint is called civilization. That's the same as living like animals, because animals always obey their base physical impulses.
Human beings are different.
They have moral rules that constrain their behavior. The point I'm making here is that, even if homosexuality did turn out to be hard-wired into the physiology, it still wouldn't begin to answer the question of whether it is moral to act on the desire. That's a different issue entirely. My point is that people seize upon scanty evidence to justify something because they want it justified, not because they've examined the issue carefully. They'll say animals practice homosexuality.