The line between erotic fiction and pornography in the U.S. is nonexistant.

There are no legal standards for "porn" as text.

OBSCENITY definition (widely googleable):
For something to be "obscene" it must be shown that the average person, applying contemporary community standards and viewing the material as a whole, would find (1) that the work appeals predominantly to "prurient" interest; (2) that it depicts or describes sexual conduct in a patently offensive way; and (3) that it lacks serious literary, artistic, political or scientific value.

(Emphasis added.)
Anything grammatical and reasonably well-written can be argued to have "literary" value, and almost anything else can have "artistic" value.

"Pornography" has *no* legal definition. "Child Pornography" is:
any visual depiction, including any photograph, film, video, picture, or computer or computer-generated image or picture, whether made or produced by electronic, mechanical, or other means, of sexually explicit conduct... (followed by vague description of "sexually explicit." However, it does have to involve a minor child, which fictional characters are not.)

Note: Pictures. Child porn=PIC, not text.

I'm not sure I think *any* text legally needs to be tagged "Unavailable for minors."