I'm not sure where this whole thing about white men preferring white women comes from. I've always felt that in America it was women with darker skin that were glorified - not extremely dark but certainly not Caucasian. To be fair, I have known a lot of white guys that were in no way attracted to black women or Asian women, specifically, but most wouldn't rate a white woman as the top choice either - not a guy raised in the states.
If there is a bias in the dating arena, it's most likely due to familiarity and opportunity. Other than that, it may just be that white men don't find African-Americans or Latin-Americans attractive - but most likely more due to their perceived personality traits rather than their looks.
Take these men abroad and they'll have a field day.
I'm white, and it took me moving to Asia to find white skin attractive, though I don't really look for it on white broads. lol My point is that white skin is definitely not considered the ideal in American culture. If it was, why would white women be so bent on cooking their skin into a different color? Asians think we're crazy for doing this, especially considering that they, unlike us, do value white skin on an aesthetic level.
I'll also say that I don't think finding a certain race unattractive makes you racist either. If someone can prefer a particular race for dating or sex without being racist, than it isn't racist to prefer not to date or have sex with that same race, in my honest opinion.
If preference is on a sliding scale, there'll naturally be two ends of the spectrum.