Consider the following, almost to a person those on the left would agree it was (would be) OK to vote for a man for the President on account of his being Black, or for a woman (say like Ms Clinton, but never never like Ms Palin) on account of her being female, or for a student to be admitted on account of his/her gender, race or other superficial feature.
This is wrong and here is why. I will use “vote” and “Black” as my examples, but this can be extended without modification to other situations and criteria (like admissions and gender or race).
Premise 1: It is a permissible criteria to vote for a man because he is Black.
Premise 2: If a thing is a valid criteria for, then it is valid criteria against. Valid criteria compromise those things you consider when making a choice. If a thing is a valid criteria then it is purely personal preference whether a particular criteria weighs for or against a decision.
Statement 1: It is racist to vote against a man because he is Black. Conservatives get told this all the time, it must be true.
This is a direct contradiction, of the two premises. Therefore either premise one is false or premise two is false. Premise two is not false by symmetry (and kindergarten, “turnabout is fair play”). Therefore premise one must be false.
If you voted for Obama because he was Black or support affirmative action, therefore you are a racist. Logic insists. See.
Or to put it another way, Martin Luther King said the color of a man’s skin was not a valid criteria with with to judge him. Why was he wrong?