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i think it adds a meta element to it in which the viewer is a participant in the porn, and as such adds a new element that makes it interesting again.
you notice it wasn't just an anime girl getting fucked by a nog? it was a personal address to you that she's going to do that. that's the entirety of the 'cuck' thing online. there's also an element of detachment, the whores in porn that are telling you how much they want to fuck someone else AREN'T ACTUALLY your significant other. its all a game of fantasy. why blacks specifically? well most people still see them as big disgusting missing links so there's a bestial element to it where she's like 'i'm forgoing all social convention and going to fuck this gorilla'.