Because it's how I feel. I can't just be a girlfriend. I won't be happy thinking he can leave at any time. I don't believe in sex before marriage, it's an inherent belief. I do it, I've done it, but it's a sin. I don't want to be sinning my entire life. I can still get pregnant, I do not want even an accidental bastard baby. I believe in the sanctity of marriage. That doesn't apply to girlfriends.