Well, xstoryplayer comes pretty close, and from what I have been reading regarding updates and progress on the next version, it could be one step closer.
For me the perfect sex game would be a modable open world where you can create and edit characters. New positions could be unlocked as you progress or achieve certain milestones. Npcs could have different preferences, which will lead to more positions being used in more ways, rather than having a huge list if positions that are hardly used. Relationship building with the npcs would also be required, no jumping right in and fucking anything that moves. Also, one price for the full game, no micro or monthly payment crap