It's not quite impossible. He could have roundly blamed everything on James, casting Lily as a pure, victimized, agency-less casualty of his manipulations. That seems to be what Snape does.

Awww... but that puts Harry in an impossible position. There's nothing he could have said that would have worked. If he had said Lily made a good choice, that would have directly insulted Snape. And Snape must have known that Harry would be in an impossible position, so he must have wanted to trap Harry into thinking he had done something wrong.

It is possible to respond in a less direct way. Competent social skills would suggest putting very little actual content into his answer. I am less cunning than Harry and not known to be particularly conservative in my expression of potentially provocative positions but even so I do not think I would have all that much trouble responding with tact. Snape would still make a hostile reply but he does that to answers to potions problems anyway. He probably wouldn't be tempted to kill me.

I don't know about you, but having someone tell me I should give up something I've been overfocused on for a long time can be quite painful.