Lots of people have suffered from sexual assault, or know someone who have. Lots of people have also been enslaved, sometimes for sexual purposes. Very few people have ever been attacked by an axe-wielding goblin or by a fire-breathing dragon.
And? How does including such things in D&D affect those people? Should the creators of D&D products try to avoid any topics that might trigger something negative in anyone who might read it?
This is not to say that an individual DM shouldn't adapt their game to the individual players involved, or that WotC shouldn't have some degree of sensitivity to what they put into print. But context is important, and further, I always come back to the question: how far do you want to go in an attempt to protect people? (More on that in a moment).
Lots of people have been "bled dry" by "vampires." Lots of people face "dragons" and "axe-wielding goblins." It is clothed in fiction, in mythic forms, but they can relate to anyone's experience.
Or is it the literal nature of slavery, of breeding that you find problematic? And therefore, must D&D fantasy be completely mythic, with no connection to history whatsoever?
When you include standard-issue D&D violence in a game, the players are usually armed and armored and on more-or-less equal footing with their foes.
When you include sex slavery in a game, you are overwhelmingly affecting only one gender (especially when it includes forced breeding, like with muls) who is almost invariably rendered helpless to the slaver's whims--and by slaver, I mean the GM.
What about war veterans, most with some degree of combat-related PTSD?
These things exist in real life, so why not fantasy? Again, it is one thing to glorify them, quite another to just present them as part of a world within which they make sense - and again, within a fantasy context. I do not think presenting them in a fantasy story is necessarily harmful to someone who can connect their experience to it.
I mean, I understand that this approach is based on good intentions: trying to protect people from experiencing uncomfortable things. But I think it is misguided, in a similar way that over-protecting children from any possible harm, say, keeping them inside from the germs in dirt or the possibility of being kidnapped, actually ends up causing more harm than good. Obviously some protection is necessary, but it can go too far and, I think, has gone too far for many, whether with regards to child-raising or "safe-spacing" around something as relatively innocuous as a fantasy role-playing game.
A DM who really feels the need to can still include it in their game--and judging by places such as r/rpghorrorstories, there are plenty of GMs who feel the need to. But there's no reason why it has to be part of the actual canon, and you don't lose anything by saying "muls are a fertile species" rather than saying, as the original DS books did, that "muls are sterile who are deliberately created by owners who forced the mul's mother to be raped and as such, are all born into slavery themselves."
When it comes to muls, it literally is carved in stone. That's the only way muls are made in DS.
And on that very note, you can add whatever sort of slavery you want in your games; you don't have to feel limited by what's in the books.
Muls are a rare instance of something being "carved in stone" in such a way. But if a person doesn't want that back-story they don't have to play a mul. Or if they want to, the DM can accommodate that. And of course, not everyone has to play Dark Sun! Should it, then, not exist, except in a format that removes any possible triggers?
It doesn't really matter to me re: something so specific as muls, at least not in a practical way (that is, I can find a way to play the game however I want, no matter how it is published). What I am concerned with is what seems like unreasonable and unnecessary restrictions around what can or cannot be published, based on how it might affect people, based upon their background. First of all, the line seems to continually be changing, and the circle narrowing. Secondly, I honestly think that, despite the intentions behind it (mostly good), it often causes more harm than good.
Again, I understand and practice the underlying ethic: to be aware of the feelings of others, to "do no harm." But again, I see it as misguided; at the least, going too far (in some cases, way too far) with every tightening restrictions; at most, it actually causes harm.
Have you ever heard of exposure therapy? It has been used to generally good results with war veterans and other trauma survivors. The basic idea is exposing them to a small dose of what traumatized them, be it through memory or virtual reality. This is not to say that D&D should be a therapeutic environment, but I bring this up to point out that encountering such things in the safe context of a fantasy game with friends may actually be beneficial.