(this is actually the type of thing I worried about as a bizarrely religious preteen/early teenager. I knew I wasn't having sex, so theoretically I would be safe, but what if I experienced virgin conception, like Mary did? Would anyone believe me? So terrifying. Finally I realized that Mary was special because she was conceived and lived without sin, whereas I was a bad-tempered kid who did horrible things like lying to her parents and hitting kids at school. Thus, while abstinence isn't 100% effective at preventing conception, abstinence PLUS regularly disobeying one's parents is!).