Well, the original use that I have seen was a brand put on movies and later books and other things indicating that the discretion required to decide whether or not to view the picture required a certain maturity of mind. The irony, of course, is that once you've acheived that maturity of mind, you aren't all that interested in simple prurience. But that's another matter.
From that useage the term slip-slid its way to mean "suitable only for adults to decide about" and eventually, in the mindless way of language, came to mean "containing raunchy language or images".
The really mature threads on the Mudcat are things like Rick's stuff, some of the fiction threads, "Why We Sing" and other heart-felt exchanges in friendship, and others of like quality.
The raunchy stuff isn't mature in the original sense of the word, it's just occasionally funny, but more often fatuous.