When I first began gaining gay friends (if you don't have any, you should, gay culture is uber fun) I asked one of them about it. I had always been under the impression that being gay was not a choice for the same reason as lot of people (I didn't choose to be straight, I simiply don't find men sexualy attractive), but I wanted his perspective. The answer he gave was this:
"Why would I choose this? Half the people out there hate me that haven't even met me. My life would be a lot easier if was straight, but hey, I love cock."
To suggest that sexual orientation is a choice you are not only saying someone chooses who they are attracted to, but also who they ARE NOT attracted to. None of my gay friends want to bumb uglies with the opposite sex at all. Most of them have, but as I can imagine, it would be about as uncomfortable as me "trying out" sex with another man.
I don't need anyone to give me any scientific evidence that being gay is innate. I believe my friends when they tell me, as I can't fathom a reason for them to lie.