Instincts
An astonishing number of adults, and even teenagers, manage to get by without either getting pregnant or getting someone else pregnant, despite what you seem to think is an irresistable urge to have sex.
As far as teaching kids goes, there is a big difference between teaching them strict morals, along with the facts they need to know, and having them maybe mess up anyway (Lord knows it happens), or assuming they're just going to screw like minks no matter what you say, leaving a basket of condoms on the kitchen counter, and having them mess up by default.