Of course there's always the [stereotypical] notion that the black man, heavily muscled with his huge endowment, is the evil, dirty, villainous fiend 'taking advantage' of the innocent, tame, demure white damsel with her husband 'powerless' to stop him, yet is secretly turned on watching his wife be 'taken'.
No other fantasy scenario plays out quite as well, though they may be pretty good in their own right.
Seems like every woman has seen those 1 or 2 white guys that are bigger than most big black guys, but across the board more black guys are possibly bigger than white guys...or something like that.
I can merely speculate, given that I'm not walking around checking cocks and shit.
Besides, it's not like any can be as BEAUTIFUL as mine, anyway.