I believe there is medical evidence that indicates there exists a genetic predisposition to alcohol addiction.
That does not excuse such a person from going through life drunk.
That being said, no such medical evidence exists for a genetic predisposition to homosexuality.
Which wouldn't be the same thing as being "born gay" anyhow. But this simply means that no doctor has pin pointed a "gay gene." Nature, nurture, and that mysterious thing we call free will all have a place; it has been shown, I believe, that out of identical twins separated at birth there is a far higher chance that any "gay" proclivities seen in one will appear in the other, than would exist between fraternal twins.