In the USA and UK fictional child sex stories are not considered child pornography but they are prosecuted under obscenity laws, you still go to jail and end up in the sex offenders registry.

In Canada and Australia is even worse, they prosecute owning these fictional stories under child pornography laws.

For context, and as a warning:

United States of America Plaintiff - Appellee v. Frank Russell McCoy Defendant - Appellant (2017)

"McCoy wrote, posted, and emailed links to stories about the rape, torture, and murder of children. He was convicted of transporting obscene matters, in violation of 18 U.S.C. § 1462, and sentenced to 18 months' imprisonment and two years' supervised release."

https://caselaw.findlaw.com/court/us-8th-circuit/1769060.html