Is sexual orientation inborn or is it a personal choice? Where does the line between nature and nurture blur?

For an elective summer class (I need extra credits to graduate), I'm taking a class called "Gay and lesbian culture in America." My professor, a lesbian (FWIW), told us that there is a debate going on about sexual orientation. She said that a minority of the LGBT community says sexual orientation is a choice and this pisses a lot of people off. Being a straight guy, I don't think about this very often. Is there scientific evidence that suggests that sexual orientation is simply inborn? Is that debate still going on?

P.S. Not sure if x-posting to r/ainbow would be a good idea.

Edit: Wow, people really didn't like this question.

Edit 2: Scientific evidence. Yeah, I agree with your arguments. I'm pro-LGBT. How come redditors are pro-science for everything else, but this is taboo? I'm just trying to better understand something I haven't thought much about.