You're not taught to breathe, crawl, eat, cry, or many other activities. You don't need to be, because they're naturally imprinted on all of us, without the need to be "learned".
Correct. And my brother snores, and I don't. My eldest brother barely crawled before he walked, whereas I spent ages on all fours. I eat very slowly, my brother was dubbed 'garbage disposal machine' by my father due to the rate at which he ate.

Do I need to go on? The wonderful thing about 'naturalness' is that it has an innate and fantastic diversity. I like women, what's to stop one of my brothers liking men, and why would it matter if he did?

Moreover, why do you feel that the state is barely allowed inside your wallet, but want it involved in the legality of whether two adult, rational human beings can form a legal bond?