Interesting question!
Race play is something that I would never ask for in a session. The awful history of slavery and its terrible impact on those who experienced it would completely prevent me from getting into subspace. I am fine, hell I am even turned on by some degrading language and objectification in a session. Being called a slut or whore or any language along those lines makes for a hotter session for me, but even that isn't necessarily something I would request in a session. When it happens by surprise it is even more exciting.
The death and horror associated with slavery, or as JB noted in his post regarding the use of Nazi symbols is just too real for me to ever want to see it in a session.
But! I do realize and accept that others might have an opposite opinion and I can totally respect that, even if I don't understand it.
jack