In regard to your question in food for though - NONE, as long as it fits with a story line.

I am bit sorry to see this discussion turning into finger pointing while the originally, well intended Moby made a valid point: why can we not find a story where Afro-American is well educated, well situated, well endowed sophisticated sadist giving it tastefully to little white whore? Thinking now about it, I did find more than one of this scenarios but in F/f story - not so much with Males as Dominants.

It will be a sad day when our fantasies get censored, and I hope not to see it.

There are codes and story outlines (which some authors use better than others), but in the end the bottom line is - if you don’t like it - don’t read it!
And, pretty please, leave my n/c, young and incest alone - I love jerking off to those!