Well, I'd say they put the carriage before the horse and mixed the cause and the effect: "geeky" students tend to be less popular, less attractive, [thus suffering in personal life] - but more successful academically [that's what makes them "geeky"]. Thus it may be not abstinence that contributes to academic success, but rather academic success that contributes to abstinence...
Exactly. I am surprised that Heritage did not control for the most obvious variable: IQ. It's well known that IQ correlates significantly with age at initiation of sexual activity. It's entirely possible that the average IQ of the abstaining group is significantly higher than the sexually active group. Which would explain its greater academic success and career aspirations.
Not that I am opposed to encouraging abstinence. I am only opposed to social science that ignores the obvious.