It should come as no surprise that many children don't cherish and honor the sexual part of their lives with respect and responsibility. These all-important aspects of sexuality are left out of their sexual education and are instead countered with shame and avoidance. Even more progressive sex-ed programs don't counter the negative consequences of sexual activity with the positive consequences — consequences that simply cannot be realized unless one is educated, possesses a positive self-image, and has the social skills to differentiate between a good partner and a bad partner.
Obviously these kids are not waiting "till death do us part" to get busy. Even adults know that one's wedding night is not a good time to find out about your wife's psychopathic obsession with flannel or that your new husband is no more endowed now than when he was born. It does these kids no favor to keep them in the dark or even in the shadow of the light. We can't keep expecting them to make sound adult decisions about an adult activity when we limit the amount and kind of information they're given. These kids aren't stupid — they're undereducated. Many of them would be right to guess that many of their classmates don't have an STD, yet. The girls just might get away with the unfounded notion that they can't get pregnant right after or right before their period, for a while. In their minds and with the limited knowledge they have, what else is there to know?
There's plenty more to know — but who's telling them?