I think Buzzfloyd's suggestion is valid. The term "adult" is ambiguous. I don't want an intelligent conversation like the one we are having about sexual preference to be interrupted by someone telling us that we shouldn't talk about it because children could be reading.

In fact, I would go so far as to say that while members of all ages are welcome, we do discuss mature subjects and as a general rule, do not believe in censorship. Regardless of your chronological age, you are expected to behave like an adult when you are here.

I would call porn, porn.