Of all of the AI safety concerns, I think this is one of the least compelling. If an LLM veered into this kind of topic out of nowhere it could be very disturbing for the user, but in this case it's exactly what they are searching for. I'm pretty sure for any given disturbing topic you can find hundreds of fully-written fictional stories on places like AO3 anyways. I mean, if you want to, you can also engage in these fantasies with other people in erotic role-play, and taboo fetishes are not exactly new. Even if it is illicit in some jurisdictions (No clue, not a lawyer) it is ultimately victimless and unenforceable, so I doubt that dissuades most people.
Sure it's rather disturbing, but personally I find lots of things that are very legal and not particularly taboo to be disturbing, and I still don't see a problem if people want to indulge in it, as long as I'm not forced to be involved.