I'm not sure how upset I'd be but I would make a mental note. I suspect he doesn't confront his mother period regardless of the subject matter. Some kids who are raised in strict households even when they become adults never stand up to their parents. In other instances if the parents have money the adult kids try to avoid doing anything that may cause them to be "cut out". The real question for you is, "Do I want to marry a man who is afraid to stand up to his mother and defend his choice for a wife?"
(You are not going to change the nature of his relationship with his mother.) When you marry someone you are marrying into their family. You have to think in terms of future family gatherings, dropping off grand-children, and lots of other times you will have to deal with this woman. You will never forget what she said to you. At least you know how she feels about you and if you go forward with the marriage you also know what you have to deal with. As I stated earlier you are not going to be able change how your husband deals with his mother. She doesn't want you for a daughter-in-law and I suspect you don't want her for a mother-in-law. It's tough enough to make marriage work without starting off with these types of issues. You have to decide if it's worth it to you.