Something like Huniepop is best for that.Take, for instance, a hypothetical match-3 puzzle game.
The problem with Sex that is too integrated into the Gameplay is you could be too focused on the Gameplay to really experience the Sex. You want the result to be a good fapping experience. Capping it off as a reward at the end is one way to delineate things.
I think the best puzzle games are Way" or Haro that has it's own structure and pacing to the gameplay and sex scene that creates the best experience for me.