I've found this thread interesting because, although I have played with and played characters who have held varying degrees of discriminatory views, I've never really thought about the degree to which such might make someone uncomfortable, so it's been very educational in that respect.
There has long been a policy on games I've helped run about the way in which we expect our R-rated, limited consent games to deal with violence. It says:
The game is rated R for in-scene language and adult situations. Also, be aware of the general tone of the game. We do deal with some darker subject matters, however, we ask that you be sensitive to other players and be sure that violence and explicit situations are a part of character development through RP rather than incidental, gratuitous, or continuous.
Additionally, we ask that out of courtesy for your fellow players, you place a warning on any explicit scenes or text, particularly in issues of abuse.
We wrote this policy after we had some players RP violence and abuse sort of as an offhand 'I'm bored let's do something' type of situation - no follow-through, no character growth, no real consequences of any sort. That's not the sort of game we were, or wanted to be. But we also didn't want to say that you couldn't deal with those themes at all, so this policy grew.
For me, adding 'discrimination' to this list seems like a good start in terms of addressing how I'd expect to see it handled. Don't walk into a bar and start spouting off racist terms for shock value. Be sensitive to making other players uncomfortable, both in language and in content. Clearly label logs that deal heavily with this subject matter. Subtler forms of -isms should lend themselves to character and story growth and should have consequences (for both sides of the equation) and should not exist in a vacuum. And it shouldn't be something that comes up almost every time you RP a scene.
Some of this is easier for me because I don't run historical games, where the consequences or frequency might be murkier, but I think the people who have been saying that there's a clear difference between an asshole who insists on their right to use the n-word or refuses to off-cam scenes and the person who plays a character who wants to play off the tension and struggle and drama the presence of an -ism sometimes provides are in the right ball park.
) and he invited me to a Skype channel where we were chatting about theme, and when the question of horror came up one example that came up was a PrP based around a woman miscarrying a child. Someone put up a lot of objections about how might trigger people, and that we shouldn't permit that kind of plot to be ran, which ultimately led me to decide to not be part of that process.