I disagree with the less is more approach. I can see where you're coming from, but I think it's possible to go all out hardcore and make it work, if only we think about how.
The goal is a pornographic game, so therefore finding a concept wherein sex is a game, a challenge, is probably a good idea. But not that stupid trivia shit, because then you're not getting sexy stuff until you've already won. First idea that comes to mind, which will of course only make the concept even more taboo, is rape. But not like RapeLay, because it should be an action game.
I figure it's more like a fighting game or beat em up, genres that can easily fit into all the criteria listed above. Of course you're not doing simple fighting moves, the entire game is about sex, and all the moves reflect this. It's not that you're doing kicks that somehow rip clothes off, it's that you're doing grabs, grabbing at the clothes, ripping them off. And if the clothes are already off, you grab some titty. Grabbing the titty then increases your arousal level until you get it high enough to cum. Of course ripping the clothes off does too, but not as much. Basically, instead of inflicting damage on your opponent's health meter, you're raising your character's arousal level. Grab a titty, goes up 5 points. Finger a pussy, another 10. Penetrate with penis, 15, and each thrust is another 3, +5% for anal.
Perhaps since the game is already about rape, your opponent does have a health meter as well, but this is a secondary meter that has more to do with your final score rather than simply passing. If you want anything better than a D Rank, you gotta drain this meter before filling your own. Or perhaps the closer you get the meter to the bottom without killing the opponent, the higher your rank. Killing your opponent results in a loss, unless you're doing a special necrophilia mission or something.
Of course, draining the damage meter is itself difficult until you learn to play well enough to not be a one pump chump. And you certainly don't want to prematurely ejaculate, which is definitely a loss and gets you no reward (unless maybe you're a cuck who gets off by having your intended rape victims laugh at you). I don't know how exactly to incorporate this into gameplay though. Suffice to say, under normal conditions, draining the damage meter causes the arousal meter to fill up faster, so arousal will max out before health empties. Perhaps there are certain difficult to pull off maneuvers that don't fall victim to this ratio. Perhaps there is a dedicated "don't cum" button that you have to mash to lower your arousal level. Perhaps there are RPG elements that control how aroused your character gets by certain moves or certain enemy traits, or how aroused you can get in general before climaxing, not to mention plenty of other stats that other regular games alter with RPG elements.
In addition to the health meter, the opponent also has her own arousal meter. Hardest to raise, but gives you the biggest points reward, and is of course essential for S Ranks.
Everything written above works just as well with a female player character, only swap the situation so that rather than the difficulties of the meters being
player arousal