The problem with these discussions is that people try and protect their interests by defining an argument out of existence. That would be fine if you were talking to right-wingers, but this is a leftist discussion list.
I don't really have a strong opinion as to where homosexuality "comes from" because, as the data you cite suggest, I don't know of anything which correlates adequately to homosexual preference or practice to form the basis for such and argument. I do think that there is a lot of evidence that there are consistent trends in male sexuality that show in both gay and straight men.
As clear as it is to me that there is not a continuum of gender, it's equally clear that there is a continuum of sexual preference and practice. I think the idea that different sexual practices create different "gender identities" is based on the idea that a person who crosses a line of taboo must be fundamentally different from a person who doesn't. I don't know of any evidence that's true.
The instinct that clearly does exist is the instinct to have sex. It doesn't exist in children. It develops in adults with sexual maturity (by and large). I think that the answer to the conundrum of why this instinct is so variously expressed will be found to lie in the fact that the instinct forms well after a very large amount of mental develpoment and socialization have already happened. Because the desire to have sex comes so late in the human life cycle compared to other animals, individual humans can come to associate very different things with sexual arousal. Nevertheless, it's difficult to contend that the desire to have sex is not an instinct.
boddi