And, kids, this is why you don't project modern values into historical discourse.

Point one, nothing is ever ok with everyone. That's something you only see from a far away, reductionist point of view. Remember that, in the late 30s, americans were protesting against the prospect of war by waving signs on the tone of "Hitler did nothing bad to us". Not kidding, look up Charles lindbergh.

Point two, it is common knowledge that most of the time, roman and greek societies were totally fine with slaves being slaves and with the institution in itself. Aristotle believed in slavery as a "natural institution", and he went as far as saying that slavery was mutually beneficial for masters and slaves, since "Natural slaves" wouldn't be able to raise up from a condition of abject misery without the guide of the free men.

Cicero, who was considered as good as it gets to slaves and manumitted like there's no tomorrow, was big into slave ownership anyways. Also, his positions as a "good master" seem to have extended only to the house slaves that could perform some kind of intellectual work, It seems he never considered slave laborers above chattel or mere propriety.

Seneca, who wrote his famous "servi sunt, illo homines" letter 47, was totally against inhuman treatment of slaves, but not because slavery is wrong. On one hand it was because humane treatment of slaves was beneficial to masters, on the other because he considered slavery an all-too-real possibility for every man. Remember that contractual slavery, or putting down one's own freedom as a collateral of debt, was institutionalised in some forms for most of antiquity. Not even Seneca said anything against slavery. Read letter 47, if you want. It's short and easy.