My boyfriends mother is prejudiced,& unreasonable, what should I do?

There isn't much you can do. You can't control anyone's behavior but your own. It's good that you are venting here. She is just one of those people who like to find what "ons to push" and then smash them for all they are worth just to get a rise out of you. I don't know why they do this - some sort of pathetic attempt at being powerful when they are truly at the bottom of the heap and very powerless and lack any sort of control. Or, she is just rude and plain stupid. You can't do anything about this. About the only thing you can do when she starts going off on things, is say, "Why would you say that?". Or just, smile, ignore her and keep things very superficial with her.