First of all, there is nothing wrong with taking advice from your parents. Parents usually share a special bond with their children and you and your parents are doing the same. As you said, you are the only child of your parents which makes you close to your parents and that’s beautiful. You should actually take advice from your parents as they are much older and more experienced than you.
As you are an only child, it’s obvious that your parents are always concerned for you and they might get protective sometimes but that doesn’t mean you are a mama’s boy. And this term is actually not right. If a son takes her mother’s advice in every life decision and respects her choices and opinions then he is not a mama’s boy but he knows how to respect her instead.
Taking your own life decision by yourself is great but it’s great when you involve your parents in it too. No doubt, not all parents and their children share the same bond and that is understandable too. But if you share a special bond with your parents and take their advice and you enjoy it, then it doesn’t have to make sense to your girlfriend.
And that’s so mean of your girlfriend for calling your respect and love for your parents by calling you a mama’s boy.