I actually think that the sex system is somewhat advanced compared to similar games out there - just discovered this game last week and really enjoy it so far.
I had a similar problem with one of my slaves - she had full loyalty and she was getting a lust warning message. Sex was initiated through consent, but because she also was uncivilised, she resisted with brute force, as obedience is still less than 80 at that stage.
Now that does not make any sense to me at all, but neither does female logic haha