Written explicit content generally makes it through. Mailroom staff are primarily looking for specific types of problematic content, not adult material between consenting parties. The things that get flagged and confiscated are communications related to the inmate's current criminal case, anything that looks like running a business from inside the facility, threatening or violent language, anything that mentions contraband, escape-related content, or material that reads like terrorist or extremist communication.
Sexually explicit written stories between adults are not on that list at most facilities. The mailroom is not reading mail to police the personal lives of inmates. They are looking for security threats.
One caveat worth knowing: explicit photographs or printed visual material can be handled differently than text, and some facilities have policies against obscene images even if written content is permitted. But text-based content of an adult nature directed between two adults is generally going to reach the inmate.