Why Librarians?

What is it that makes the thought of kinky librarians such a turn-on? A quote from Ezra Klein:

I'd submit that sexual craziness from unexpected sources is far better than wild actions from the obviously unbalanced. It's the librarian thing -- having the demure break out the whips is infinitely more erotic than having the girl who's been talking about whips all night do it. The former's an achievement, the latter's exactly what you expected.