I think that genetics plays a role in most, environment in others, and for a very select few it is a choice.
I went to a performing arts high school and it was very common there (and very accepted) to be a gay, lesbian, or bi. I knew someone who pretended to be gay, and he later told me, so he could fit in better with the dance majors. I also knew two guys who I felt may be gay, but denied it due to their religion. He said he chose to be straight to avoid judgement, but he recently came out, and he is happier than ever.
I agree with the poster that said they believe love can be between twon individuals regardless of age, gender, or race. I personally hate labels, I think they prevent so much.