Ahhh...the bastardization of classic terminology.
nature, n. 1.) The inherent or essential quality or constitution of a thing.
Unnatural in this context has nothing to do with prevalence in (Mother) Nature, i.e. the animal world. It is not natural for a human to breathe underwater. It is not natural for a human to fly. It is not natural for a human kidney to be able to filter all the salt out of seawater. Yet all of those qualities occur in other organisms.
The reason those things are not "natural" is that they are not part of the essential constitution of man, even though they might occur in other living things.
Let's go back to Darwin. Why did the Reproductive system evolve? To Reproduce. It's not the Pleasure system, it's not the Degrade-myself system, it's not the Be-cool-in-Hollywood system. The nature of the genitals is to generate. You cannot postulate any model of Darwinian selection whereby these organs come about for any reason other than reproduction. It is simply not possible biologically.
Homosexuality, like all sexual perversion (bestiality, masturbation, etc.), is unnatural precisely because it denies the true theological and evolutionary nature of the genitals: reproduction.