Nature is about life and the continuation of life is it not, we've evolved into what we are because of it. You are saying it's a "natural" behaviour which is a completely different sense to the word. Nature has no bearing on our behaviour? So i still think it goes against nature. Don't get me wrong i don't want to ban gay people or outlaw it, it's just in "my opinion" to be wrong according to nature.
Nature is not about the continuation of life, it's about balance. Population gets too large, population is reduced.
Ever thought that this is nature's way of controlling a population which it deems is expanding too rapidly?