While it already is considered taboo for a white or black man to marry a bi-racial woman (at least in most places), most of the pressure currently comes from the family and community of the white or black party, not from the bi-racial partner's.
I'm sorry but I must be leading a very sheltered life but I have never heard of anyone in my circle of family (family mostly mixed black hertiage with inlaws of a other races) and friends (all races) saying that they would disapprove of a family member marrying "that mixed girl". Hell alot of my cousins are even more mixed than me, so your theory goes out the window.
Where are you getting this crap?
Josie
P.S. nevermind being of mixed heritage myself I find I'm not interested what your sources are ~ I live the life already, I'm not on the outside looking in