Because so many people have touched upon many of the same points in their comments, I decided to compose a single post addressing some of the most common concerns and observations. As time permits (I'm not online at home, except to read text-only via cell phone) I will reply to individual comments on other points that were raised.
Also see:
"Prejudice against Muggles and Muggleborns," "On using the term 'racism,'" and "Ethical considerations in the scene in Snape's Worst Memory" are discussed behind the cut:
( The prejudice against Severus, based on class, appearance, House affiliation, and so on, exhibited in this scene is just as morally and ethically objectionable as the prejudice against Lily based on her circumstances of birth... )
If we want to apply real-world concepts of prejudice to the wizarding world, then let us see this scenario (and the five-year history leading up to it) for what it really is: divide and conquer. One reason oppression continues in the real world is because the various oppressed groups are set against each other. To see Lily alone as right and Severus alone as wrong is to miss the bigger picture of multiple bigotries that interweave and permeate the social and relational dynamics at Hogwarts and in the larger wizarding world--and in our own. Racism, sexism, classism, status-ism, affluence-ism, beauty-ism: It's all connected. None is better, or worse, than another. None should be set against another. Tug at one string in the web of life, and everything begins to unravel.