I've wondered about this myself:
I'm wondering whether the word "ADULT" is becoming a completely new word, which no longer means what it once meant.
In fact, I would be willing to bet that in a lot of communities (whether real life or online), the word "ADULT" alone is a red flag.
If you doubt me, try putting a neon sign in your front window with nothing more than the word "ADULT." A Bud or Corona sign is one thing, but neighbors would call the cops at the sight of that filthy word, and something would be done.
ADULT has come to mean filth, raunch, sex.
The same thing's happened to mature, as in "this film has grossout humor involving noisy and disgusting bodily functions and gratuitous sex scenes a fifth-grader would be embarrassed to snicker at and is intended for mature audiences." As an aside, Roger Ebert (I think, paraphrasing) once remarked that the biggest change in movies in his lifetime was that young people use to go to them to learn how to imitate adult behavior, and now adults go them to learn how to imitate youthful be.