I've only skimmed parts of this topic, but I'd like to go ahead and throw in a couple cents.
A lot of porn games do a bad job of integrating their porn into their gameplay loop. I've played great games that are poor porn because of this and horrible games that are great porn because of it. The main difference is frequency and duration. You don't want to spend hours waiting for the porn, but you also don't want the game to interrupt you every ten minutes for an hour long porn scene. Games that have sex tied to a core mechanic of their gameplay loop typically do very well at keeping a steady feed of smut rather than this uncomfortable stop and go that most game that don't seem to have.
Although I do think it's the easiest way, sex doesn't strictly need to be a gameplay mechanic itself. It's ultimately about pace. Those NPC rpg maker games are by no means good games, but they actually integrate their scenes pretty well in the loop. Usually you just explore, maybe do a puzzle, find a girl, watch a smut cutscene, and repeat. It's not amazing gameplay, but it's easy to fap to.
Conversely, there are rpg maker games that set you out on a dragon quest or your final fantasy, then interrupt two hours later to stop the game with a twenty minute sex scene for one CG (+variations). The porn is basically not a part of the gameplay loop with these kinds of games. If I want to play a game, this is much better, but it's not great for fapping.
There's a happy middle ground between these extremes, but it's clearly quite easy to screw up your pacing if sex isn't a game mechanic.
That said, having sex as a game mechanic doesn't mean it's very well tied to the gameplay loop. I recently played Flying Princess Interbreed, and while it had a breeding mechanic and great art, the mechanic was poorly implemented. To start sex, you intentionally had to give up a turn and it resulted in the loss one of your units for the remainder of the stage. This implementation was so punishing that it normally encouraged you to avoid engaging with the mechanic at all.
A lot of porn games do a bad job of integrating their porn into their gameplay loop. I've played great games that are poor porn because of this and horrible games that are great porn because of it. The main difference is frequency and duration. You don't want to spend hours waiting for the porn, but you also don't want the game to interrupt you every ten minutes for an hour long porn scene. Games that have sex tied to a core mechanic of their gameplay loop typically do very well at keeping a steady feed of smut rather than this uncomfortable stop and go that most game that don't seem to have.
Although I do think it's the easiest way, sex doesn't strictly need to be a gameplay mechanic itself. It's ultimately about pace. Those NPC rpg maker games are by no means good games, but they actually integrate their scenes pretty well in the loop. Usually you just explore, maybe do a puzzle, find a girl, watch a smut cutscene, and repeat. It's not amazing gameplay, but it's easy to fap to.
Conversely, there are rpg maker games that set you out on a dragon quest or your final fantasy, then interrupt two hours later to stop the game with a twenty minute sex scene for one CG (+variations). The porn is basically not a part of the gameplay loop with these kinds of games. If I want to play a game, this is much better, but it's not great for fapping.
There's a happy middle ground between these extremes, but it's clearly quite easy to screw up your pacing if sex isn't a game mechanic.
That said, having sex as a game mechanic doesn't mean it's very well tied to the gameplay loop. I recently played Flying Princess Interbreed, and while it had a breeding mechanic and great art, the mechanic was poorly implemented. To start sex, you intentionally had to give up a turn and it resulted in the loss one of your units for the remainder of the stage. This implementation was so punishing that it normally encouraged you to avoid engaging with the mechanic at all.