I'm a white guy, and I'm a sucker for dark-skinned girls (especially black, but any skin tone darker than mine is already a turn-on).
I believe we humans have an instinct of looking for a mate that is different than ourselves, be that difference in appearence or personality (that's why they say that opposites are attracted to each other). It just so happens that "race", as you call it, is one of the most visible differences between people, at least one of the first we notice.
But that, of course, is just a general tendency, I'm not saying that everyone is attracted to people of different "races", just that it probably is the case most of the time. Of course, this is only my opinion...