I agree nocomp. There are many white boys already and more will follow, that if given a direct choice between either fucking a pretty white female themselves or giving up that chance and instead watching a black man fuck her they would choose the latter.
Even after watching it take place they would still be ok with the decision they made to pass on the chance to fuck her and be an observer of her getting blacked instead. Especially if say all 3 were in a room full of people afterwards, the white boy, the pretty white female and the black guy that has just fucked her. Even the humiliation would be a turn on for the white boy with his thoughts of the black guy now holding an advantage over him.
The advantage being he has had that pussy. He knows what it's like, how good it is, he has been inside that pretty little thing standing beside him. They are standing over there together. Him looking confident, his black dick still coated slick with her juices and her laughing and having a good time looking sexually content as she carries his black cum inside her.
He has had her but I haven't. I don't know how good that pussy is but he does. He might tell me if I were to just ask him. It is amazing just how much sexual thrill that some thoughts alone can produce and deliver.