One day almost two years ago I just sort of realized that I liked guys as well, and not only girls.
When I look back at the time before that, I can see instances where I was experiencing attraction towards males, but I was lying to myself in order to convince myself that I was straight. I would think things like "While that guy is cute, I can't possibly like him, since I like girls. If I were a chick, though, I totally would". I didn't think much about that you could in fact like both.