Right, and, if the text stories are deemed obscene by the court, under regular obscenity law, yes you can be prosecuted and convicted, but it happens only rarely. See the Frank McCoy and Karen Fletcher aka Red Rose cases. Contrary to popular belief, it was never ruled that text-only material can’t be obscene; it just became generally unenforced after the adoption of more restrictive tests for obscenity and the explosion in visual porn.
Heck, one could imagine that a local jury could make a ruling of obscenity against a written porn story because it does have minor characters in the story, if the judge does not instruct them otherwise. But the fact is that the presence of minor characters does NOT make a story per se obscene.
As Shalmanese points out, PROTECT Act prosecutions for “non real” materials have never made it to SCouUS and are ALSO relatively rare, usually piled on when the accused has already called unwanted attention upon himself for some other reason, as an additional charge to pile on.