“Disgust relies on moral obtuseness. It is possible to view another human being as a slimy slug or a piece of revolting trash only if one has never made a serious good-faith attempt to see the world through that person’s eyes or to experience that person’s feelings. Disgust imputes to the other a subhuman nature. How, by contrast, do we ever become able to see one another as human? Only through the exercise of imagination.”