There have been a large number of Great Debates threads on this topic.
Most of us (myself included) feel we were born gay, and that this is an unchangeable aspect of our selves. Contrary to myths that suggest that it’s somehow a result of upbringing (sexual abuse, passive fathers, etc) we actually come from a remarkably diverse range of family, cultural, and religious backgrounds. The fact that ex-gay programs don’t seem to work lends credence to this intuitive understanding.
Science has tended to back us up, at least in the early research. The twins study suggested a strong genetic factor – about 50% of gay men with an identical twin had a gay twin, while the percentage was much lower for fraternal twins, and only about 1% for adoptive siblings of the same age. Contrary to what one poster said, though , the other 50% wasn’t necessarily nurture – hormones in the womb could also play a role, as could other factors. Another study found homosexuals were much more likely to have homosexual relatives on the mother’s side of the family than the father’s, suggesting something on the X chromosome.
Simon LaVey’s famous dissection-of-the-hypothalamus study found that that part of the brain (which is associated, among other things, with sexuality) was a different size between gay men’s brains and straight men’s brains. The gay hypothalamus was the same size as a woman’s. LaVey’s study was considered flawed because of its small sample size, and because all the gay men had died of AIDS.
More recently, researchers in Tokyo removed a gene from a kind of fly, and discovered that the fly ceased to recognize a difference between the sexes – flies with the gene were heterosexual, flies without it were bisexual.
Preliminary work, to be sure. But few people doubt that heterosexual sexual desires are inborn – I think most people assume that if a human being were sexless robots, it would still have a sense of sexual desire. And we also know a little bit of the stunning diversity of the human animal – hair colour, eye clour, height, shape, traits of all kinds. We even know that a genetic trait can lie dormant for generations, particularly of recessive one. Strangely, we as a civilization have trouble applying all this knowledge to homosexuality – centuries of conditioning saying it’s wrong have biased our interpretation, and we assume it’s upbringing – bad parenting.
The weight meanwhile of the first objective data reinforces the subjective experience of most of us – it’s how we were born.
That being said, I don’t think it’s relevant to the politics of the issue. I don’t see any value in playing for pity – “Puh-lease! Don’t hurt us! It’s not our fault!” This is a human rights issue for me, not a scientific one, and I approach it as an academic question.