It's not just seeing him balls deep inside of her. It's knowing his big cock is enjoying a part is her pussy that my little dick can never touch. Even if he is just 4 inches long, her knows pleasure inside of my wife that I never will.
It's knowing that at that moment he is experiencing pleasure with my wife that only he and other men ever can. That is their pussy, because it can never be mine.
It's knowing that when he buries his cock in her and cums she gets to experience feeling him throb and pulse inside of her in a place she could never feel my dick in, even if she was willing to fuck me.