But it's disingenuous to take a vastly unrealistic setting - cherry-picking bits here and there of "medieval Europe", slapping in new ones that never existed, and excising realistic elements that are boring or distasteful - and then insisting that elements A and B are there because realism demands it. As you note, it's a fantasy setting. So when a GM is happy to have wizards throwing fireballs, open borders, and a distinct lack of enforced monotheism, but insists that women can't be fighters or a lifted skirt clouds men's minds without fail or consequence, well. That's not actually a game with a historically-accurate setting. That's a game where the GM wants particular gender roles, but won't cop to it, instead letting History take the rap. (Poor History!)
Yeah, it's pretty ridiculous to insist that historical realism demands anything in this crazy game. There is another sort of "realism" that is worth consideration, though: literary verisimilitude. Simply put, fictional cultures ought to have flaws for the same reasons fictional characters ought to have flaws, because perfection is unrelatable and imperfection generates drama. And sexism and other forms of bigotry are certainly near the top of the list of common cultural flaws. That said, literary verisimilitude doesn't demand that a culture has any one
particular flaw any more than it demands a character does. If your character is, say, a psychopath, and your audience dislikes that, you can't say, "But my character
has to be a psychopath, because he has to have a flaw!" You can always just pick a different flaw. Same goes for culture. Sexism is one way to give a culture that rough texture of imperfection, but there are other ways, and you should pick the way that isn't going to anger, annoy, or hurt your audience.
Turning back to the actual subject of the discussion: it's a little difficult to get players to stick around for a game when the game itself, or its setting, takes away from their fun. And for rather a lot of people, "your PC is going to have to put up with the same crap as you do in real life! because realism!" is not something that makes a game sound like a fabulous way to spend a weekend.
Yeah, a good rule of thumb is that, regardless of the flaws and prejudices in your setting's culture, the PCs should generally be exempt from it unless the player actively signals that they want it to be a part of their character's conflict. This goes for gender, race, orientation, and everything else. And it's not just ducking the issue to avoid hurt feelings (although to be clear: that is the
most important reason for doing it, since this is supposed to be a fun recreational activity for everyone). There is some in-universe logic to a sexist lord not being quite as swinish towards a heavily-armed Amazon as he is towards commoner women, or a half-orc who slew a rampaging dragon earning a respect not afforded to his kin.
