A recent study shows that abstincence-only sex ed did not reduce the incidence of teen sex. In fact, it seems to have increased it:
The study showed about 23 percent of ninth-grade girls, typically 13 to 14 years old, had sex before receiving abstinence education. After taking the course, 29 percent of the girls in the same group said they had had sex.
Boys in the tenth grade, about 14 to 15 years old, showed a more marked increase, from 24 percent to 39 percent, after receiving abstinence education.