I think the NPC on NPC rape doesn't work any more. I had a stone house dedicated to this. It contained males (two natives and a young man, IIRC) who all had the Mean personality and liked the Shameless type, and females who had the Shameless type but disliked the Mean type. It also had too few beds (as they raise moral), and a sprinkling of libido pots.
It worked perfectly but then, I think after one of the early updates, it stopped working entirely. I experimented to try to get it working again, but nothing I did worked. I eventually put the NPCs in a trunk and converted that house to something else.
I don't know if NPC on NPC rape was broken, or they deliberately removed it, but I don't think it works any more. I haven't tried again for the last few updates I admit. They will still use toilet girls at least.