Freddy: I dare say the crack about pornography is in one of the Mencken books on my shelf, but only two of them have indexes and it is not in either.

It is perhaps worth noting, however, that the OED s.v. waltz has a citation from an 1825 art handbook: "_Waltz_, in Music, the name of a riotous and indecent German dance". I gather that the indecency in those days, tho, was not attributed to the 3/4 time, but to the fact that the dancers were in physical contact.