I'm gonna guess they were talking about Child Bride (
http://www.imdb.com/title/tt0029989/-- damned if I can remember how to do links), a 1930s exploitation film by roadshow master Kroger Babb. From what I've read, it plays the familiar old exploitation-film trick of using a strong moral stance against the subject matter as an excuse to put it on screen for the thrill seekers... so, given the subject matter, I can understand why the boys at MST3K opted not to use it.
Which DVD is the interview on, by the way?