Anybody hurt in that particular case? No.
But the man opens himself up to a world of hurt should the girl regret her decision, and the law is in place to just give a clear-cut line where it's not any use in saying "but he/she wanted it too!" for the adult. It's to stop adults from taking advantage of young people.
Trying to argue the case that kids should be open prey to adults as long as the kids say yes because Bonobo chimps? Oh how far MMO-C has fallen...