How do we frame this in such a way that a white, western male might have the same visceral and instinctive revulsion to a dark campaign element like chattel slavery that, while realistic, can be offputting and denigrating to players? This is difficult to do, considering the protective ruling class bubble that colonialism, historical revisionism, etc. provide, but I'll give it a shot:
1) A nation has suffered a protracted, seemingly endless war. Returning soldiers, mostly men, have been largely abandoned by the crown to live in squalor and mental decay . The terrors and specters of war have become so collectively profound that they've manifested as actual monsters, stalking the alleys and gulches of the capital. Suicide is so prevalent among returning soldiers that a new demigod of death has arisen to embody it.
2) A nation of minotaurs keeps white human males (the paler the better) as sexual chattel, raising and breeding them over generations to rape. They're like pets, to be displayed wherever the proud minotaurs travel in their mercenary companies.
3) All younger brothers of the first born male are made into eunuchs to insure an uncomplicated line of patrilineal heritage. This is common practice.
4) When they come of age, men are judged by the dominant religion to be either worthy of conjugal relations with women or made into a second class of males. They cannot marry, the are barred from types of military service and women who respect their own social status avoid being in their presence.
Fun, no?/s
If you add any, or a few, of these elements to a game I can assure you your players will give you long, sideways glances and mutter "what the fu..." under their breath. Even if you added these scenarios with the intent of addressing them as the inhumane abuses they are - as "maturely as possible" - many players would simply not want to go along for the ride. And they'd be right to do so.
I'm more open to a case being made for non-chattel slavery (ie. prisoners of war, convicts, interlopers) like the Drow art suggests, but that would have to be outlined by a DM with zen like sensitivity.