I think sexual orientation is something that develops over the years, rather than something that instantly happens and stays that way. That doesn't mean you can suddenly say you want to change orientation and stay that way in a matter of seconds though.
I started off as being curious about females and starting to kind of lean towards them, although I wasn't interested in relationships as of the time. I thought penises looked wierd at first. I had found it hard to find any males attractive and I wasn't even sure if I was even attracted to them at all. But when I did find what was attractive in guys, my mind changed a whole lot about them. I tend to lean a lot towards guys with medium-long to long hair (head hair, not body hair), anything shorter, I find isn't really "EWW UGLY" but rather...boring I guess, like I don't feel anything. The sad thing is a lot of guys cut their hair boringly short, and it seems to be some kind of socially unacceptable thing towards the public for guys to have long hair like as if it's gross and unhygenic. Stupid double standards. Ok I'm starting to fall off-topic here...
Back to females, I can find them attractive but I don't feel as inclined to be in a relationship with them, if not at all, I'm reluctant about experimenting because being with a male sounds more fun. I guess it's best to say I'm a bi leaning hetero.