I haven't played many of these games, simply because I didn't enjoy the few I tried years ago. That said, I think there is a pretty easy solution, although still isn't something I'd play tho...
You play as the dungeon overseer, or perhaps some sort of God. There is a group of heroines who are continuously attempting to invade the dungeon, and each time they are stopped, and raped, by your minions. Afterwards, the group escapes, grows stronger and tries again. Creating the game loop and difficulty curve.
Granted, on paper this seems like a strategy, management or tower defense game. AFAIK most games lose/rape games tend to be side scroller action platformers no? You can still use that genre for this idea, however instead of playing as the heroine, you play as several of the monsters. Each monster could have different mechanics and such to make the game more interesting.
All in all, it can remain the same game from a different perspective. The heroine remains the protagonist, and you are simply the observer attempting to block her path. It remains as a lose/fuck game, where the player never loses.