Pornographic video games are actually not quite the market you may expect. They actually aren’t that big because if you got an AO (Adults Only which is essentially X Rated for games) most mass retailers wouldn’t distribute it the longest time. There’s also a lot of vestigial moralism about them which has lead to some being outright banned for their content that you don’t see in film or books anymore.
I remember lots of movies rated “R” back in 1969 that are now rated PG, or PG-13.