Replying to Science behind being born gay?

Is there any science behind being born gay? We had a discussion on gay marriage today during class, and the teacher was saying that science proves that people are born gay, and it is not a conscious choice to be gay.

While I believe that people are born gay, and it is not a choice, I wanted to see these scientific facts and studies that my teacher was talking about, and can't seem to find any.

Does anybody have any information about this?