The "take one for the team" bit is true if we are talking about an intelligent species, but if we're talking about, say, homosexual dogs, they would have no incentive to reproduce, and simply wouldn't do it.
As such, if you ask me, homosexuality is always a choice, conscious or unconscious. But Hades' links seem to indicate otherwise, so what do I know.
Anyway, I still think homosexuality is a perfectly valid choice, for someone.