Basically I want to draw your attention to this: opportunities for sexual encounters are derived from social interactions; the more people you meet and the more you talk and meet up with the people you know, the greater your chances of locating a person willing to enlist you amongst their romantic prospects.
Therefore, sex is a result of popularity, which explains some of the intelligence rating questions.
If you are very, very dim you probably have no social skills and have few friends, and thus meet up with few people. When you do, you probably have very little of interest to say and nobody thinks you are particularly attractive.
However, if you are very, very smart, you probably also have a restricted social group. This is because the elitism of the upper echelons of cleverness prevents them from forming a wide circle of associates with similar interests, et cetera. In other words, if you are a geek you are likely to engage in some graceless enthusiasm about a particular area of geekiness which only a limited range of others also get all bouncy about. A lot of people who have no particular interests in anything intelligent (those from the middle marsh of intelligence ratings) will assume you boring and unsociable because they cannot indulge in the same topics that you do. Thus the range of available partners is reduced, so the likelihood of sexual advancement is less and sexual realisation is delayed.
I also want to point out that this does not mean being a geek means being less sexy (as is the common stereotype). This just means that very few people are prepared to accept that you could be sexy because of the overwhelming crime you have committed by being a geek.
To be honest, I think everyone already knew this, they just weren't thinking about the situation properly because it either seemed better to use intelligence as an excuse for virginity or because they wanted to assert moral reasoning behind choice of abstinence. Ok, fair enough, but the statistics must depend to some degree on lack of choice, because smart teens are often just as horny as average teens. True, concentrating on work can lead to lack of interest (I revised for about 400 hours for my A-level exams: I KNOW that feeling), but it doesn't crush it completely. Verily, clever teenagers lack the opportunity and biological pressure to obey their reproductive instincts because social occasions just don't arise with the same kind of frequency.