Late to the party, but I'm going to chime in here. I'm white and have bulled for a white couple. I'm no "white Mandingo" or anything, but I'm bigger than average. More importantly though is that I am a Primal Dom in the kink world, and the wife wanted that treatment.

She couldn't get it from her husband, who either doesn't have it in him or from whom it wouldn't be believable to her enough to get off on. I think you're right though as I type this, because the common trope of a black guy being the bull implies, because of your blackness, you are assumed to be a big dicked rampaging beast.

What this all comes from, IMO, is the feminization and sissification of western men (specifically, white men). So these white chicks end up marrying a guy that they want to be a smart, caring and productive man by day, but a dominant sexual beast in the bedroom. Today's soyboys, nu-makes, and general fucking manchild pussies don't have the dominance in them to fill that animal need in these chicks, so they seek the animal sex elsewhere.

TL;Dr - Western white males are largely pussy-ass bitches that can't satisfy a woman's natural animal lust. White women who marry these cucks racistly auto-select black men to fill the animal urge, white guys gets off (for some fucking reason) on the humiliation of his wife fucking a beast that she shouldn't.