To be fair, there are games where you come for the porn and stay for something else, though IME that something else is 100% of the time the plot. My examples are MIST, where the mystery actually got me invested, Ravager, where the lore and characters got me invested, What a legend, where the humorous story got me invested, and probably a few more but these three come to mind right now. Can't think of any where the actual gameplay got me invested though. So, yeah, I'm not sure porn game devs should focus too much on the gameplay.
There's also Monster Girl Island where the dev focused a bit too much on the lore, trying to tell some epic fantasy story about world at war and shit... kind of boring compared to my above examples, and a real waste given how amazing the 3D porn was.