Sexual Orientation Isn't a Choice?

So after this on Facebook, http://imgur.com/MUpfr I decided I would ask people that are smarter then two middle school kids that link say "let me fucking google that for you" and send me to the same link I posted. So why do people think that sexual orientation isn't a choice? I'm open minded and just want to know why, not be told how wrong I am and someone linking me to this reddit or something.