To be clear, I mean using solely the term "adult". Using the term "adult pornography" is fine.
But to answer your question: Because it is destructive and wrong, no matter the age of the viewer. Calling it "adult" not only suggests that it is fine for adults, but entices minors to it under the false impression that they will be more adult by viewing or participating in it.
But to answer your question: Because it is destructive and wrong, no matter the age of the viewer. Calling it "adult" not only suggests that it is fine for adults, but entices minors to it under the false impression that they will be more adult by viewing or participating in it.