This is one of those very interesting topics.
There seems to be a type of irrational fear behind much of the criticism of bm/ww relationships by black women. Back in high school I knew black/japanese mix girl who openly told me once that "white girls have a big advantage in getting black men because they are prettier and there are so many more of them". I was shocked that she even would say something like that.
I went to a mostly white university and had both white male and female friends and black male friends. There was a clear effort by black females to ex-communicate black males who dated white women. I dated a mixed race girl whose dad was black and whose mom was white and she was raised by a adoptive white family in an all white community. As soon as she got to college and began hanging out with black women she adopted the same attitude. She had previously dated mostly white men, and thought that was perfectly okay, but as soon as she saw a black male with a white women she would start complaining and getting negative. There was some deep seeded fear working with her. When white guys dated black women, most of the black males that I knew had no problem with it, or if they did not particularly like it, they did not make a big deal about it...I think because their disapproval was not based on fear. When fear gets involved the negative reaction is always sooo much more intense. Black male white female relationships seem to create much more fear in people (mostly on the part of white males and black females) whereas other types of interracial couples do not. When white males date black or asian women they are often seen as being "open minded" and tolerant. Black men and white women are not prisoners of this fear and do not seem so prone to the racism that it causes.