This was basically my response as well, so I'm glad someone else said it so I can me-too rather than trying to explain myself better.TheAbominableDan said:If it were occurring in animals how would we know?SlasherX said:It isn't. In biology it is not normal. No where else does this occur or is tolerated except as humans.
Thinking out loud, then: The reason we know homosexual animals exist is because we can observe sexual behaviour between individuals of the same gender. Could we observe trans behaviour in the wild (and infer the existence of transgenderism) in the form of cataloguing activities which are specific to males and females of a given species and seeing how often and with what degree of regularity these social gender lines are breached?
e: something just occurred to me. It sounds like the OP is prejudiced, but doesn't discriminate, since they mention having gay or trans friends. Having a prejudice isn't automatically a bad thing, but acting on it (discrimination) is.