Robert Rector of the Heritage Foundation has a useful piece [1] on National Review Online about the recent study 
showing that teen abstinence education can work. He analyses the intense oppositions to these programmes by many sex educationalists.
He says they are opposed to abstinence programmes for three main reasons:
First, abstinence programmes teach that teens should abstain from sex until they have at least finished secondary school. Ninety-one percent of parents agree. But many sex educationalists promote “comprehensive” sex-ed programmes which teach that it’s okay for teens to have sex as long as they use a condom.
Second, abstinence curricula teach that sex should be linked to “love, intimacy, and commitment” and that these qualities are most likely to be found in marriage. Again, 90 percent of parents support this message. But, Rector says opponents are “appalled at ‘privileging’ marriage over casual relationships, cohabitation, or ‘hooking up'”.
Third, such programmes teach that marriage can be beneficial to children, adults, and society. He points out he black out-of-wedlock birth rate in the U.S. now hovers at 70 percent; the overall out-of-wedlock birth rate is nearly 40 percent.
Rector continues: nothing enrages the “sex-ed advocacy industry more than telling at-risk youth that healthy marriage might be a good thing for them. (Safe bet: No sex-ed curriculum funded by the current Congress will say anything positive about marriage.)”