He is causing her mental anguish about what she was taught, though. She is thinking she was lied to, while he knows people just did not know any better. And as he seems to care about her feelings...
They are his slaves. If he cared about their feelings, he would se them free from slavery and then try to have them remain as free partners, out of their own will and for their own benefit. What he actually cares about is the relaxing atmosphere and harmony, for his own sake, plus making the slaves stronger and more skillful, and thus more useful for him, as the owner. He spends hard days in the dungeon, risking his life to a certain degree, so it makes sense he wants to recharge in peace at home afterwards. He obviously doesn't want to feel like a rapist either, at least so far, so I'd say for the sake of his own mental health, he prefers to pretend sex with sex slaves is consensual enough.
So, yeah, he does go a certain length to make the slaves content. However, that's like a Porsche owner taking really good care of his sports car. You could say it's for the car's sake, but in reality it's for the owner's own sake since the car is his prized possession. Slaves are a slave owner's pricey possessions.
