I assume you got that from the other thread going on right now...
Natural is the improper descriptor for a conversation about sexual behavior. Anything that happens in nature is natural. It's that simple. Anything that's man-made isn't natural.
Short list of natural things...
Dogs, trees, water, dirt, rape, murder, love, homosexuality, horses
Short list of unnatural things....
Cars, cell phones, International Space Station, Polonium
What people who call homosexuality unnatural are really looking for is the term "immoral". They are trying to say it's immoral because their bible tells them it is and they think it's yucky. Only then can we have an honest discussion about homosexuality and society.