I would contend that virginity is not necessarily tied to intelligence, but personality and experiences.
I think your last sentence sums it up best.
Also, I agree that there sample was pretty small. I think their goal was not to conduct an accurate statistical view of virginity compared to major, but more or less appeal to the stereotype that smarter people have less sex. I was just watching TV about 30 minutes ago and they had a new special on how teens are deciding whether or not to be sexy or smart. They see celebrities like Britney Spears, Lindsey Lohan, etc. and think this isn't just appropriate, but actually the way you should act. Perhaps there is a correlation.