MyOwnMuse
That Crazy Cat Lady
About a year ago(sometime in the beginning of June, 2005), several scientists published a paper wherein they changed a single gene in a male fruit fly, and discovered that the fruit fly was suddenly courting only other male fruit flies (they did the same to a female fruit fly and got the same results).
You can find several covers of the article in places like the nytimes online if you have a subscription, as well as a bunch of other on-line newspapers if you hit up google the right way. I would post a link, but my newness as a member is preventing me from being able to do so.
Of course, humans are increasingly more complex than fruit flies, so there's no exact translation to be made here, but I think it is something that can be taken into consideration. Fruit flies are common subjects for genetic testing, and a lot of experiments done with them have lead to a better understanding of our own genes.
I personally believe that both genetics and the environment have an effect on sexuality, but that it is certainly not something that happens by choice.
By the way, if anyone has any updates in the fruit fly experiments, I'd love to hear them.