I've never seen an erotic based Visual Novel or Game designed for multiplayer... There might be some more arcade style games out there with some adult themes perhaps, but I don't think they are like the ones you see on this site and others... Have you tried looking over the adult games listed in Steam? Maybe there might be more game oriented fare available through them...
If your just looking for normal multiplayer games of any sort, I'm sure there are a ton of those...
Probably the biggest reason you don't see any multiplayer fare with erotic content, is because it would be extremely difficult and far too complex to make a multiplayer Visual Novel... Where as a game might be more in-line with that sort of thing, yet it's still gonna be far more difficult to design... Add to that, normally reading/playing these things tends to be a more private thing... Not something you expect a ton of couples to be interested in reading/playing at the same time... So without the market showing much of an expectation for it, you don't see anyone taking the extra effort that would be needed to make multiplayer in the first place...
Now, there are 3D Social Games out there, that allow multiple people to join in, mostly in order to socialize and play around... But they usually have nothing in the way of actual game play or story based content at all... They are just adult themed social environments... Check the site below for a list of some:
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There is also
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, which has zones designed only for adults... It's another 3D social environment, but it is a huge sandbox environment, where players can make all sorts of stuff and even sell stuff they have made for in-game money that translates to real life money... Some players have used that system to create game like elements, but you have to explore and find them...
Most of those sites are not entirely free, depending on the company model and what you want to be able to do in them...
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