Namely because teen abstinence stops abortion and venereal disease in its tracks, both money-making industries for the Dem Party and their liberal allies (think Planned Parenthood).
Sex Education is equivalent to giving a classroom full of middle-schoolers guns and ammunition and relying on a 40-minute gun safety video to reduce fatalities.
"So remember children, when you lock the magazine, retract and release the slide, flip off the safety and pull the trigger, be sure your guns are pointed in a safe direction."
It's like the Teen Healthy Habits program handing out packs of cigarettes, after making scrupulously sure that they all have filter tips.