“In both cases, it is the prejudice, not the condition, that does the harm. It may be, as some would have it, that blacks are inherently inferior to whites or that homosexuals are all, by definition, sick. So what? Even if either condition truly is inherently undesirable, no manner of social pressure will turn blacks into whites or gays into straights. Social pressure will only exaggerate the handicap. It is still the prejudice, more than the condition, that does the harm.”