This is a world where slavery is absolutely fine. That 'noble' queen allows it If we were to take the realities of this isekai into consideration, I'm sure you'd come to a different answer.
The thing is, Naofumi is a "current day"-person - so his morality is different from the world he is currently in. I don't even know what you mean when you say "to take the realities of this isekai into consideration".

You don't go to Iran and say "oh, stoning people is fine here - guess I'll do it too - after all, when in Rome, do as the Romans do...". Or what do you mean by that?