First, I want to talk about the broad spectrum of "arbitrary human prejudice". I'll define this as "any time a human makes an assumption about another human in one area based on information about them in an unrelated area". For instance: "You are old, so I assume you don't know how to use Instagram" or "You are a man, so I assume you can change a tire" or "you are gay, so I assume you have good taste in clothing" or "you like the Yankees, so I assume you are a jerk".
Humans make these kinds of correlations, both negative and positive, of "If you are X you are also Y", based on pretty much EVERYTHING. Not just gender/race/religion/sexual orientation/class but even more trivial and random, like whether they're fans of video games or sports, of Star Trek or Star Wars, of fantasy or westerns, of romance or action. Whether they like knitting or crocheting, what kind of clothing they wear, what kind of makeup they wear, how they style their hair. There is nothing that humans do that is so minor that somewhere, someone isn't making a connection between it and unrelated behaviors.
Making connections between seemingly unrelated things is what humans do. Some of these connections are highly usefu, some of them are highly annoying, and a lot of them are somewhere in between.
Beyond that, disliking people who are Not Like Us is also an innate human behavior, observable even in infants over something as trivial as food preference. (This is such a depressing study. I hope it turns out to be wrong or unreplicable or something.)
I am pretty bored with fiction that explores oppression based in gender/class/orientation/race/religion/e
I don't particularly want prejudice in the books I read. But if an author presents me with a society that is "just like medieval Europe, but without gender roles or ethnic stereotypes", my first thought is "that's not going to be 'just like'." And if you write it as if it is, it's going to feel implausible, as if you're not writing about humans at all. If there are no ethnic stereotypes, why do the ethnicities persist instead of everyone just blending into one ethnicity? If they're highly dependent on manual labor, why are men and women doing the same jobs at the same rates regardless of whether their physiques are suited to it? I'm not saying these are unanswerable questions -- only that I want answers stated, or at least hinted at.
It's not that I want oppression because it's realistic. I read f&sf. I do not require realism. But I want my dragons and mages and unicorns to be internallly consistent and well-thought-out. Not just "dragons because dragons are cool!" but dragons who do X, Y and Z and have impacted society in ways A, B and C, etc. I want the causes and implications of an oppression-free society to also be considered. The human beings I know about from history and my world are pretty terrible at it. I'd love to know why they're better at it in a fictional setting.
OTOH, I don't give it any more consideration than I give the assorted fantastic elements of a story. When a movie or a short story happens to have a diverse cast with no visible prejudices, I don't care if it explains the why of it, because that format doesn't lend itself to explaining all the details of the backstory.
But I wrote an epic fantasy, Prophecy, my unpublished first finished draft, where I did lots of world building. But I literally never explained why there are basically no ethnic tensions or sexism. It's a low-tech world, reliant on manual labor, with no sophisticated forms of transportation or labor. This is not a situation where global open-mindedness has historically flourished. It feels sloppy to have gone "because I said so" on the topic.
I'm only giving fantasy examples because, eg, if the setting has good communication and transportation, where most work is intellectual rather than physical, then it makes sense to me that stereotypes that arose from distance and physical differences will die out. It's only when the society looks analagous to an Earth one that's loaded with discrimination that I will go "huh. Weird that you didn't get that here."
Still, overall, I am probably more inclined to overlook "because I said so" as the explanation for a bias-free society than I am to overlook other inconsistencies in a setting. I've read a lot about prejudiced societies and lived in one my whole life. It's refreshing to just skip all of it. But I still like it better if there are hints about how the fictional society managed to escape it.