If Homosexuality was based on the genes, then it would be bred out of the gene pool, due to the lower reproduction rate among gays.
IMO, in the cases where they "are born that way" (as opposed to the ones who were molested etc) it probably occurs in the womb. Now what causes this? Something the mother is taking? Recent studies show babies in the womb are being subjected to HUNDREDS of artificial chemicals.
I would also consider the mothers being in some way traumatized while pregnant, and releasing messed up hormones into the baby. For example, studies show German children born immediately after WWII (when their mothers were mass raped by Jewish led Soviets) have the HIGHEST documented cases of homosexuality in Europe.
Feel free to add your opinion.