Far before the Victorian era, a woman /girl was valued for her virginity above all other things. I believe there is a deeply rooted cultural and biological component to this. Did society first impose this bias or did the it first arise from instinct? We are not so far removed from our primitive beginnings.

Practically speaking, I would tell you DD that kids start to experiment sexually (even if just by rumor) because they have sexual feelings, because they want to act older, because it is forbiddden, because they think that is how you show love....and because they think it's cool.