I agree that this is somehow pushing some button of your mom's.
You can just privately feel sympathetic for her, that something is still getting to her and she has such a hangup, and then ignore her and leave the house. (You don't have to make her agree with you; you don't even have to answer her question. She says, "why are you dragging your BF along?" and you say, "Bye, Mom--I should be back about 9:30." And leave.)
Let her have her hangup. Just don't make her comments become YOUR problem--they're not. Think of the grownups in the Charlie Brown movies (wah wah wah-wah-wah). In one ear, out the other, and do NOT let it stop in the middle anywhere. "There goes Mom again."
Or you can immediately start quizzing her about what is it that bothers her so much, almost like you're her therapist, and then offer her sympathy. In other words, completely subtract YOU from the conversation and make it be ALL about her and her hangups. That'll be unpleasant enough, maybe she'll stop.