You capture a new slave, they come in at -100 relationship.
If you tell one of your other slaves that they can fuck the new slave, the new slave's relationship goes to -120, which I expect is supposed to be capped at -100. It's not a big deal, because if you go annoy the new slave again, it will pop back to -100.
A bigger problem is that one of my long-term slaves had his submission reset to 0. I think it happened after I brought in my first female slave. He got horny, and I told him we'd share the new slave. It said something about we had reached a new understanding, so I hope it isn't an intended effect, but the next time I looked at him I noticed his submission was 0, so that wiped out lots of work.