I was raised extremely liberally in a church that was full of LGBT, pagans, humanists, Jews, Christians, Buddhists= a Unitarian Universalist church- although I think it might have been of a very liberal variety. Everyone was often drunk at church events, but as kids to preteens, we had sexual education that was of a very accepting, open, question variety. So I, even as a kid enchanted about science fiction and aliens, became open to the possibility of falling in love with someone else, regardless of the gender or appearance or anything other than them as a person and personality. I was interested in girls like any other lustful boy growing into a man, and I once questioned my sexuality like everyone does at one point or another and couldn't find a relationship with a male that gave me the same goosebump heartache that chasing after a girl felt like. Then halfway through high school, I was out of it at a party and was ambushed by a guy, and I was mid-blackout at that point and this suddenly woke me up and I kissed back and felt sparks, the same I felt like when I was thinking about a girl, and we didn't do much but that and kind of grinded against each other. So after that, I 'experimented' or said let me see how this goes with a few more guys, some good some bad - but same as girls. So I then decided.
Long story short, if you want to skip the above, it was partially environment/upbringing, ideas introduced in childhood that grow into concepts in adolescence, and then experience, I never would have known had that guy not kissed me (or something else happened in the future along those lines). So no choosing here.
