Being born gay doesn't just mean that you came out of your mom's pussy and wished it was your dad's asshole. It can mean when you were born you were introduced into an environment which can lead to the probability of homosexuality. Like Q said, hormones will jumpstart homosexuality the same way it stops the notion of girls having cooties. Is it a choice when a man have a wife and three kids but he's a closet homosexual? Even if the only thing he wanted was that his dick would get erect to the sight of boobs instead of dicks, how often do they TRULY 'convert'?

And yes, there's gay DNA. It's those cells in your penis that look like Mr. Rogers' face.