There is very conclusive proof that it is. I'll try to find the artical. It was in Scientific American, or Discover, something like that. Said that there was an area of the brain that was formed differently than heteros that affect sexuality. You cannot cure what is, in effect, a genetic variation.
As far as empirical studies go, Alfred Kinsey determined that

sic) In the field of sexuality identification, everyone falls somewhere on a scale. 1 being completely hetero, & 5 being completely gay. Most people are towards one end, or the other, while many of those that fall nearer the center live a life denying their inclinations
BTW, I've known 2 men who are 100 % homosexual. One is flaming, the other is not. The guy who is not went through a series of relationships & decided to become celibate after finding out most other gays are drama queens, LOL. He has no interest in children & he's baby sat his nieces & nephews many times. The guy who is flaming, ironically, went to college, got married, settled down, & had 3 kids. Then he came to the realization he was living a lie & accepted his sexuality. He is still a friend to his ex & a good father to his girls.
I've known other gays, but not really well. I think many gays are fucked up, not because they are gay, but because all their lives they are forced to live a life that is not just denying themselves, but actually having to publicly condemn what they are in reality.