The history of sex education in the public schools is this simple: After contraception and "safe sex" began to be "taught," the rate of teen-agers who became pregnant out-of-wedlock, as well as the rate of STDs, skyrocketed.
At the same time, due to a variety of factors, including skyrocking taxes and the women's movement, mothers began working outside the home in large numbers. This removed the hands-on parental influence from kids' lives for much of the day. This, in turn, resulted in kids raising kids, rather than parents raising kids.
Those two factors have gone hand in hand toward this current situation. IMO.