[You're making false assumptions - not everyone has that kind of willpower (To resist teenage sex), and many teenagers certainly don't.]

Just like fat people can't resist eating, smokers can't resist smokers, drinkers can't resist the sauce, murderers can't resist going all stabbie, rapists can't resist violating people. For serious crimes people who can't resist their harmones or urges or whatever we put them in jail. For ordinary people who can't resist their mundane urges we do nothing because they only hurt themselves. Not that I actually believe anyone out there can't resist their urges. This kind of troublesome premarital sex has risen over the years which means that previously they controlled their urges and presently they do not. That means cultural changes are to blame, not some overwhelming genetic urge.