Ustwo

Oh I'm firmly on the track, I think we are on different trains

both currently and historically - who have practised on a wide scale and condoned same sex behaviour and orientation (such as the Sambia or ancient Greeks),

In my last post, which you may not have seen I pointed out this is bisexuality not homosexuality. Bi sexuality, especially learned bisexuality, wouldn't stop you have having children. Its really an apple vrs an orange argument here. I don't have anything to cite by my own memory, but from what I was taught, homosexuality was not considered normal or desirable in the greeks. You had social sex with your male friends, and wives, slave women, and prostitutes for the real thing.

the "birth defect" argument assumes that homosexuality is caused by genetics.

No, not all birth defects are genetic. Birth defect means the damage was done prior to birth. It doesn't matter if its genetic or environmental at that point.

Upbringing may be the predominant role,

From what I have read, this has been pretty well debunked.

exposure to various hormones at certain stages of development might do it,

This may well be true to exposure to hormones prior to birth in male homosexuals, it doesn't seem to be true after birth. The 'male' brain is stamped into place and doesn't require hormones to be male after birth. This is why young boys like to hit things with sticks, and are less verbal than young girls. Neither has sex hormones telling them what to do or their brain how to form at this point, the change has already occurred. Young boys who act more feminine have been shown to have a greater chance of being homosexuals once they reach sexual maturity than more masculine boys. I can't say how this applies to female homosexuality as its less well studied. One thing that was done was to treat male homosexuals with testosterone to see if that 'cured' them of being gay. What the result was they were more horny but for homosexual sex, the hormone didn't make them straight, their brains were not wired at such.

or there may be a "gay" gene.

There MIGHT be a single 'gay' gene, but I don't think thats the case. I think there is most likely a group of genes which makes one susceptible to being gay. If the mothers hormonal condition is right (and the leading cause is believed to be stress hormones in the mother) and you are susceptible to being gay, the brain turns 'gay'. It doesn't matter a whole lot if its a gene, gene's, or pure development to the child, it all happens before birth and can't be changed after by any means we are aware of.

The reason I think its genetic is there does seem to be a family trend for it. Families who I know have homosexual members, tend to have other family members who do as well, though not always immediate family (which would help to limit the 'nurture' argument).

You're focusing on only one potential cause, without considering the other strong possibilites, which is hardly a very scientific or dispassionate way of looking at things as you claim to do.

I have been nothing but scientific or dispassionate. I think the issue is you don't have a good understanding of the science and are not thinking through the situation. Most people think of genetics as a black and white science. Bonde hair or brown, blue eyes or green, all based on the old dominant/recessive genes you were taught in biology freshman year in highschool. In reality very few genes follow this pattern, and there are many overlapping effects. If there was a 'gay' gene, my guess is it would have died out a long time ago. If there was no genetic component, then we would be able to isolate what the environmental conditions were that caused the change. Instead I think its clear we have a genetic predisposition which can be affected by the conditions in utero.

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