As far as I know, once you have asked them for intimacy in the meeting phase of interaction and they say yes, you can never again rape that slave. You can only rape someone by going straight to the sex, rather than meet, portion of interaction and choosing to have sex with someone in the list whose name is red.
Looked a bit further into it. It seems the forced counter is only used by the old sex system which is no longer in use. So, yes, it looks like it will currently always remain zero.
