I don't think it's selfish. I do however, think it's a bit extreme. When we feel passionately about something, we sometimes tend to lean towards drastic actions.
Your decision not not have children is your alone and if you feel comfortable in that then no one should ever be allowed to pressure you into doing something you don't want to do. However, there are plenty of ways to avoid pregnancies without making an irreversible decision. Like Skipper said, you'd be just as safe with condoms or other forms of birth control.