The bottom line is, no, sexual orientation is not a choice.
Various groups claim to be able to change sexual orientation and gender identity through therapy. There is no scientific basis for these claims, and the success rate is very dubious. Often people in such programs emerge feeling bad about themselves, and experience guilt when they are not able to change.
In fact, the American Psychological Association actually convened a task force to examine this issue. Here's what they determined after a systematic review of the peer-reviewed journal literature on sexual orientation change efforts (SOCE):
"Efforts to change sexual orientation are unlikely to be successful and involve some risk of harm, contrary to the claims of SOCE practitioners and advocates. Even though the research and clinical literature demonstrate that same-sex sexual and romantic attractions, feelings, and behaviors are normal and positive variations of human sexuality, regardless of sexual orientation identity, the task force concluded that the population that undergoes SOCE tends to have strongly conservative religious views that lead them to seek to change their sexual orientation."
Though we don't know exactly what determines gender identity or sexual orientation, we do know that neither can be attributed to any one, easily adjustable, factor, and that being gay, lesbian, bisexual or transgender it is not something you simply decide to be.
Unfortunately, not everyone agrees. As a bisexual teen writes,
"My dad asked me why I was making a stupid choice and I said "it's not a choice" and it's not. It isn't a choice as much as being straight isn't a choice. I did pull that one on my dad--that I just know and it's like knowing you're straight. My parents just don't understand and think about it the same way I do. Which I guess it makes sense because they were brought up around more people that are straight and less that were gay or bi."
This is a sadly common situation, but ultimately, what is much more important than trying to be someone who you aren't, is learning to like the person who you are!