The thought of giving up their ass to white men is repulsive to women. If you're white, and your girl says she won't fuck in her ass, what she is really saying is that she won't fuck with YOU in her ass. Given the same scenario, except with a black man, she would happily bend over, spread those smooth cheeks, and eagerly wait for his turgid black weapon to knock on her pink puckered hole. She would spend the next half hour moaning and groaning, tightly grasping the bed sheet, face blushing, neck muscles straining, teeth clenching, taking that hard black dick as he repeatedly shoves it up your girl's dirt chute. She would feel every painful, blissful, exquisite thrust in that tight hole she denied to you. She will beg for his cum inside her rectum, something unthinkable with you. And she will thank him as he leaves her in a sweaty, cummy pile when he pulls his pants up, buckles his belt and leaves without a word.
Not just wives I've done this with my mother. We've been having sex for over a decade I love watching her getting her back blown out by big black cock.