I'd say the only real exposure to any sort of adult multiplayer game I've had would be the erotic roleplaying in an MMORPG. I could never get into it because of a multitude of reasons. Typing slow, bad spelling, inconsistent quality, etc. Outside of the erp there was always a whole other AAA game to play as well. Other games like Second Life and VR Chat also exist, but those communities are small and face many of the exact same issues.
I know it sounds kind of ironic, but the worst aspect of an adult multiplayer game in my opinion would be the part where you have to play with other humans. I suppose if you designed it in a way to remove the human element then it could be fun. There are a lot of text-based sex games with fairly advanced sex mechanics. I'd look at those as a starting point. You could approach the sexual encounters in much the same way where players aren't typing erotic strings of text at each other, but are instead simply playing the game using the mechanics at their disposal while the game itself is pumping out all the actual text descriptions of what's happening.