There is another possibility too that is worth considering.

Sexuality may be, for humans, primarily social in the same way it is for Bonobos. Reproduction may simply be a functional necessity met as a side effect. If we see sex as a social interaction and the choice of a sex partner as a social matter, then the question is how various individuals fit within social structures or not.

There may be in-born aspects to how people relate to social factors and for some people these are quite evident (autistic people, for example, relate to social structures very different from non-autistic people). But even within the spectrum we might call "normal" (I don't like the idea of normal btw), there may be cognitive and even neurological reasons why some people may relate to specific social structures in specific ways.

However, beyond that, there are functionally necessary reasons why people must fit into social structures differently, and why at least some people must necessarily challenge and fall outside a social structure. Every taboo in society must have someone who challenges it. Every norm must have someone who falls outside it.

The reason is that without this process of perpetually challenging society, there are no individuals. You can't be an individual and blindly follow social norms. But beyond that perhaps on an evolutionary level, without people continually challenging, and redefining, social norms (a process which btw happens much faster in verbo-motor than literate societies), culture and society are static and dead and there is nothing to adapt it to the problems of the day.

So might be able to see various non-standard forms of sexuality (i.e. outside the social model of "normal" which in our society is defined by Barby and Ken, husband and wife, etc. i.e., monogamous and heterosexual), as necessary acts of rebellion which transform and breathe life into society. The only problem is how some Christians relate to their myths of rebellion but that's a different topic.

In this way we can perhaps past the argument as to whether gays are defective people who can't help it and simply can't conform to models of normality, or whether they are immoral people who choose to live outside accepted social norms.

I don't think that sexuality can be reduced to applied neuroscience just as chemistry can't be reduced to applied quantum physics.

One of the huge problems of course is how Christians relate to the story of the Fall from Eden. You hear this brought up all the time, and the lesson they draw from it is follow social mores. Don't rock the boat, don't rebel. But a close, careful reading suggests:

1) The fall from Eden is foreshadowed in Genesis 2 with the description of marriage.

2) The forbidden fruit is general heterosexual activity. This is particularly obvious when including a look at Middle Eastern iconography and, in particular, figures like the Pazuzu, who had a serpent-headed penis. Yes, the tree of knowledge is knowledge in the biblical sense ;-).

3) This act of rebellion creates the Biblical model of (heterosexual) marriage.

The problem for Christians is that the very model they want to push arises from the very act of rebellion they want to use to bludgeon everyone else into submission.