Well, color me stupid and call me a fool. I don't think it is "natural" either.
My ex-wife put it in the most eloquently succinct manner I've ever seen; unfortunately this involved gestures which won't translate here.
Male equipment is designed to fit female equipment, just like a key and a lock.
Homo-sex is like trying to make two keys do something, or trying to open one lock with another lock. Best I can translate her analogy without pictures.
(Yes, yes, I know that isn't the only way to have sex, spare me any graphic examples please. )
Male sexual physiology, combined with female sexual physiology, is designed to facilitate reproduction, which is the function of sex in nature. Male-male and female-female fail to produce reproduction, thus failing the obvious functionality test.
It does not fulfill the function of sex in a state of nature, therefore it is not "natural."
As previously noted by Apocalypse, whether "not natural" means the same thing as "wrong" is a much more complex question.