No, it's the version of slavery that makes it more palatable for the purposes of a game. In real life, all slavery is bad, and exists whether we like it or not. In a fantasy game, we can outright say that in this world, there is no sex slavery or rape; that it just doesn't happen. Like when Gygax decided that firearms simply couldn't work in D&D and mixing saltpeter, sulfur, and charcoal did nothing but make a stinky smoke.
Also, imagine, if you will, that the PCs are enslaved. (Let's also imagine the players consented to such a possibility occurring, which is the only way I personally would allow the PC to be enslaved.)
In Athas everyone can be a slave real quick because it is a standard sentence (justified or not) a Templar can hand out. So that is part of the setting, most players interested in playing DS probably know that chances are that they might be enslaved at one point or the other wfor whatever reason. If in doubt then session 0 helps, as always.
In fact the basic 2e module (considered by some a s railroading but it is ane excellent primer) gets the PCs enslaved real fast.
Scenario 1: House slave. The PCs now have access to improvised weapons (such as kitchen knives), can boobytrap the house, poison their owner, or possibly work to undermine their owner in other ways, if they decide to play the long game.
Scenario 2: Arena slave. The PCs now have access to real weapons, other arena slaves who might want revenge, and can possibly even rile the peasant audience into revolting, if they're charismatic enough.
Scenario 3: Forced breeding/sex slave. The PC is likely naked with no access to any weapons or armor or any other support, and no agency, unless they can convince the DM that "has vagina dentata" had always been on their character sheet. Also, this overwhelmingly affects one sex over the other.
Ok the fanged m*m* as last resort defense, that is rich. But you know, in modern times there is such a thing like being a slave limited to sex slavery, in ancient times (Late Stone age to Antique which is the baseline for most DS cultures) they did not make any differences on the "purpose" be it for work, fight in the arena, victim of sadistic psychopaths or pleasure, a slave was most ofte not categorized as being one or the other.
And while getting rid of all slavery would change quite a bit of Dark Sun, getting rid of sex slavery wouldn't change anything at all, especially if you make half-dwarfs either a fertile race or no more different than half-elves in terms of how they're made, treated, and the mother's survival chances, which I recall were really low for muls.
Its like getting rid of injuries in combat, on some topics you cannot really cherrypick, even in an idealised game scenarion.
I see it like that: No wrong in your PCs parents were forced to conceive your Character, you do not have to describe the act in pictures.
Things like that can be cleared up with a kind of social contract among grown up people. The player with the lowest threshold on what is uncomfortable in explicit speech or plot sets the communication standard.
Also, there's probably more gamers who have been sexually abused or assaulted than have been actually enslaved, so having non-sex-related slavery is likely to be far less personally upsetting than having sex-related slavery.
And i bet even more gamers have been mugged or physically assaulted in some way in their lives, and a some might have (had) substance problems, so what now? No robbers, thieves, assaults, beatings? No alcohol? I get that sexuality in games is not everyones thing and i respect that. But if kept abstract, i do not see any problems for an average grown up person if his PC heritage is from a rapelike situation because he chose Mul. Chances are high the PC does not even know his parents.
I think the phrase here is "duh." However, if half-dwarfs can only be produced by forced breeding, then that means that all PC half-dwarfs are affected by that. If I choose to play an Athasian halfling or elf or human, I can choose my parentage.
See you can choose your parents up to a point. What if your DM decrees your social status is maximum low society commoner, or if you insist that your family is one of the rich noblesse, then they got over cross with some templar at some point who enslaved them?
A nice background is one of the finer things of the game for those interested in deep RP (like me) because it gives the DM something to work with. What you do not get is fictional parents of your char that somehow are immune to the common aspects of the game world e.g. getting enslaved.
Bingo. Even our violence is sanitized and there's nothing wrong with that. D&D is not really the game I'd use to have such heavy topics as sexual assault. I don't mind if some other games touch on the subject, but for me at least, D&D is a beer & pretzels good v. evil type of game that's supposed to be fun and light hearted.
It is the personal style how people prefer to play it out on the table. Just because HP are a purely mathematical help method does not mean that you have to shun every mention of gore and such if your group is fine with it. I mean you might do it as you said, and i give you this, that D&D system is not optimised for those details, but it is in no way difficult or impossible to describe nasty things, as pure fluff.
Addendum: You do both know that halflings and thrikreen are cannibalistic in Darksun, do you?
Actually that means a halfling or thrikreen might eat a fallen comrade to not get anything to waste.
Darksun is not your simple "oh lets get 5000 Zorkmids in diamond dust somehow, and find a cleric of sufficient level to raise our comrade."