I also had a quick look on the psychology metalib pages and didn't find anything of worth. There were no papers or articles really related to causation. I didn't exactly look very thouroughly, but on the face of this there wasn;t anything really related.
However, the topic of homosexuality is much more complex than an either/or scenario concerning Nature vs Nurture. While you'll probably find countless articles relating to biological or social causes, we have to remember that behaviour is a product of both phsiological and socio-cultural influences. It's not necessarily as simple as pinning it down to a singule cause, but rather how biology and sociology interact.
Also, the APA states that sexuality is not as simple as being innate and fixed, for some sexuality is fluidic and can develop over time. They also go on to say that some people may not even begin to have bisexual/homosexual urges until later in life.
And then the question of identity has to come into play. It's not unheard of for both men and women to have homosexual experiences regardless of how they identify themselves sexually. I seem to recall one study on homophobia where one group of self admitted homophobes and a group of people who claimed to have no homophobic feelings were made to watch homosexual pornography. The researchers found that the self admitted homphobes were more aroused by the pornography than the group of non-homophobes, suggesting that identity is much more important in controlling behaviour than physiology. Identity plays a HUGE role, regardless oh what the body confesses.
I've touched on a few points, but the over-arching theme to my post is that looking at it in terms of fixed black and white isn't necessarily the best way to look at it. There's a lot more complexity, and a lot more grey areas than you'd originally assume.
