Originally it was a dislike for relationships, and my general lack of female friends.
Then, it was laziness and the lack of private places to do it in.
Then, when I left a boarding school, I got hung up on my last relationship.
After that cleared up, it was 6th form, and I was paranoid about having too much work to be able to have a relationship without screwing up my grades (I am VERY easily distracted by women, particularly women I think like me).
And here we are. I plan on being a little more out there once I get to uni. I mean, I already have friends who are girls, so it's no longer an issue of confidence around them, so it's all groovy!