It’s not always about horniness, per se. For example, I’m trans, and giving birth is something I’ll always want but probably will never get to experience. So maybe I want to experience a “power fantasy” of sorts where my trans character performs dark rituals to give birth to eldritch abominations, all so she can experience motherhood.
I’m sure there are other experiences that people would want to rp but not necessarily to experience irl. It certainly overlaps with kink, but it’s not like you’re masturbating at the table. You’re telling a story with adult themes, not (necessarily) writing erotica.
Presumably the Marshalls.