Pretty simple. Its genetic.
Proof? When do people decide they are going to not be homosexual?
When do people decide they are not going to be a homosexual? Every moment of every day when they don't choose to try to become a homosexual.
If something in one's childhood causes homosexuality (e.g. sodomized children), then it wouldn't be genetic, even though they didn't simply "decide" to be a homosexual.
Do you deny that t here are homosexuals who didn't consider themselves homosexual at some point when they decided to experiment with homosexuality?
If it's genetic, why have researchers failed to identify the genes that cause it?
How come in studies of identical twins, twins with the same genes, one twin being a homosexual doesn't establish that the other twin is also a homosexual?
When does a fat person choose to be fat? Are you saying fat people bear no responsibility for their obesity?
Yes, it is pretty simple and in many times over, the case for homosexuality being a genetic aberration fails.