You know, I would love for someone to actually do such a study --- twins, no twins, gay and not gay and delve into the issue for real.
Why?
1. To answer the question for real.
2. And hypothetically, if there was a gene that made people succeptiable to being gay, it would be likely that a vaccine could be eventually created to turn it off.
All that said, I think it is largely, if not exclusively, environment and choice.
Twin studies have been done on this topic, as well as other familial studies. I can send a journal article from 2000 that analyzed an Australian twin sample. (Journal of Personality and Social Psychology 2000, Vol. 78, No. 3, 524-536) It's not for the entirely light of heart though, it's rather heavy duty epidemiology. It also contains many references to other studies, though I am limited to what I can access here. FReepmail me if you're interested.
This new study is different because they're specifically looking for genetic similarities. Asking for identical twins is rather invaluable because, well, they already know their DNA is identical, thus they're not interested in analyzing their DNA.