I think it's one of the leftovers from slavery.
The black man was emasculated and the black woman was raped by white slavemasters.
Nowadays the lingering effects of that emasculation and the mindset it engenders among people is still rampant.
I don't believe in IR dating for the sole purpose of IR dating in itself. I don't think a person should be dating somebody of a different race BECAUSE they're a different race.
However BM/WW relationships are a rebellion against the antiquated mindset that the black man is inferior to the white man. The white woman is what is presented as the standard of beauty in European culture and if a black man is with a beautiful classy white woman, it's really a slap in the face to those twisted idelogies of black inferiority. There's nothing better about a beautiful classy white woman when compared to a beautiful classy black woman, but such a relationship challenges the perception of people, making the black man an alpha male instead of a slave.
The white man has always striven for alpha male status. The culture prides itself on being adventurous and conquerors, and though white men incorrectly think their women are the best, they also have a taste for the exotic. They have jungle fever the same as black men only European standards of beauty have curbed their appetite. WM/BW relationships only serve to reinforce the idea of the white male's alpha status. It's not a role reversal or really taboo like BM/WW relationships.
Black women get mad because they feel as though they're being slighted when a black man expresses interest in a white woman. And it's true that some black men have the game twisted, feeling as though dating a white woman is elevating their own status OR passing up a decent black girl to get with a bottom feeder white girl. I actually shake my head whenever I see a Dave Chappelle/Free Willy couple.
But the fact remains that there's far more c00nery in harboring a preference for someone whose ancestors used to rape your ancestors and who thinks you could never measure up to their cultural standard than harboring a preference for the "forbidden fruit" i.e. and pissing off racists by doing things with certain girls that would've got you lynched 100 years ago.
However I don't believe in racial preference anyway as I think it's based in ignorance, but it's whatever.