Some white men prefer black women, just as some black men prefer blond white women, or some white women prefer black men, or some black women prefer white men. The same can be said of other races, and even other non race related physical characteristics. It is not right, it is not wrong, it is just a fact.
You can question the internal motivation for such likes and dislikes, but when you do, you are probably just asking for trouble.
I have read and even heard some black men who say that the white mans urge to date the black woman is rooted in his vestigial need to dominate black women, as in slavery. I suppose this is a valid concern, albeit less likely than you might think. This would seem to be unlikely in most cases. Black women are notable in modern times for not being dominated by anyone, one of the chief complaints of black male respondents in forums as a reason to date white women, who are supposedly more pliable. Hey guys, the jokes on you! It is therefore fair to ask if the reason that many black men choose to date white women might not be, at least, similar.
The whole argument becomes irrelevant as soon as personal choice enters the picture. No party is forcing any other party to enter into a relationship with them. It is all a matter of choice.
Even if the motivation is less than pure in the strict moral sense, what is really important is what happens after the initial infatuation wears off. Does it turn into a relationship where both parties see each other as important individuals with whom to have a long term relationship?
The nitty gritty is this: No matter what draws two people together, no matter how questionable, or distasteful it might be for some others, the initial attraction is only a small part of the equation. Two people drawn together for all the wrong reasons may still develop a right relationship.