"existing in or caused by nature; not made or caused by humankind." - obvious, and that's why it's pointed out that it occurs in nature
"of or in agreement with the character or makeup of, or circumstances surrounding, someone or something." - it is natural for homosexuals
Sex or sex organs serve multiple purposes. If you don't believe this, go your whole life without jerking off. And once again, gays can reproduce. My gay uncle has 3 kids and is hardly alone in that. Just like heteros can go without reproducing. How unnatural and contrary to evolution they must be huh.