I took a Human Sexuality class in college and we watched a very sobering and to-the-point documentary. It was about the baby son of a nice family getting his penis burned off in circumcision surgery(don't ask how) only a few months after he was born. The solution to their problem came in the form of a psychologist, whom the mother had seen on TV. So they basically gave the baby a sex change operation and raised as a girl instead of a boy. Jump forward twelve lovingly girlish years later or so, and "she" starts exhibiting severe depression, confusion, and rebellion. She dressed as a boy, cut her hair, and did boy things. She didn't want to play with dolls anymore, she wanted to play sports.
It was only later that she found out that she was in fact a boy all along. This pissed her off more than anything else in the world ever could have. Later in her life, she had penis reconstruction surgery, took out the womanly parts, and lived as a man again. Due to all of the stress put on his mind and life from this change in sexuality, however, the depression did not go away, and the lost childhood did not come back.
TL;DR: Nurture has very little to do with the cards we're born with. You can try to paint over an Ace of Clubs and say it's an Ace of Hearts, but that won't change the original card underneath.
Not saying that nurture isn't important, but people of all sexualities are born that way because that is simply how their brains work, and no amount of "nurture" can change it.