Well there are plenty of injustices in the world, most of them we can do little about. If my owners gave me the ability the live a decent and meaningful life, that potentially included the opportunities to do things that would be unrealistic if I had to fend entirely for myself, then I wouldn't necessarily complain, no.
But if the state failed to protect me against mistreatment by said owners, then I would absolutely have reason to protest and demand better.
I believe having slaves is wrong, that doesn't make it a fact. I have no reason to believe there is any natural law dictating that, it's nothing but a social construct we have agreed to.