First of all, it's not a duopoly of blacks and whites. It's a continuous spectrum. In fact there is no whites. At best some form of pale skins. Ditto with blacks. No real blacks but people with variation of shades of light browns to very dark browns. So no duopoly.
According to your dogma, blacks > whites, then dark brown > light brown > dark pale > light pale, etc, but with a billion shades and arrows in between.
So this is where it gets ridiculous because there is no correlation with skin tone and dick size. Otherwise dark brown men would have the largest dicks, then light brown men, then dark pale men etc, except a billion steps. The darker the skin the bigger the cock.
Unfortunately reality is not like that. In fact there is no correlation between skin tone and dick size.
However there is dark brown men with big dicks, just as there is pale men with big dicks, OR, dark brown men with small dicks as well as pale men with small dicks.