Do you chose to be heterosexual? Could you MAKE yourself turn gay?
I think I could, for the reason Jor-El mentioned, to make some kind of statement. I think some otherwise straight people tend to have qualities mainly associated with the other gender, and they just get tired of a homophobic society telling them that they have to hide these traits in order to be accepted as "normal". In other words, they collapse under social pressure. Homophobia (outspoken homophobia, mainly) is a terrible thing. I've even been a victim of this. I'm a pretty introverted guy, I'm a musician, and I'm single, plus at the time, I was pretty small; most every person in middle school thought I was gay and treated me as such. It continued through the first two years of high school, but finally it subsided when either they grew up or just because I grew to be bigger than them. Some of my friends from back then gave up and just began to believe what everyone was telling them because it made the situation easier for them to handle, coping with truth instead of a false accusation (at least that's what they tell me).
To answer your question: I could, but I won't. I still have moral feelings about homosexuality that would keep me from doing so.