Amy Schalet has an op-ed today about parents allowing a teen to have their boyfriend/girlfriend sleep over. I think she's right that it's partially related to sex education, but I think that answer is too facile.
Another issue that she doesn't mention in her op-ed: parents' aversion to thinking about teens' sexuality may also be about incest norms. I have known Dutch young adults who had their opposite sex parent sleep with them when the parent came to visit, and that would never be done in the US as well. Many Americans would find this unusual, at minimum.
Even the most liberal parents are comfortable with their teens' having premarital sex but hope that won't be until after the teens go away to college, and they are uncomfortable teaching their teens about condoms because they're afraid that sends the wrong message that sex now is okay. One question is whether the key ingredient there is the developmental stage represented by being 18 and at college, or whether the key is being AWAY at college. Are parents of boarding school teens relatively accepting with their teens having sex at age 16 since they're out of the house?
Conversely, what many Americans find strange about virginity pledges is how they violate incest norms by drawing the wrong kind of parental attention to teens' sexuality, such as this piece also this week in the NYT. (As an aside, the author also exhibits an all/nothing view about her "purity ring", saying that for awhile she felt that she had to take it off once she had sex, thus making her sexuality even less private than it would have been in a more liberal setting.)
Certainly in some cases, there's a prudery element, but I think it's a broader cultural phenomenon about whom Americans are comfortable talking about sex with, possibly relating to incest norms, which seem to differ between here and the Netherlands.