Boyfriend shaved baby head without asking

Just what the title says. When my newborn was born, my boyfriend and his family said they needed to cut the baby’s hair because it’s like traditional in their family, but I was totally against this! I expressed to them how uncomfortable I was with the idea of anything sharp going across my daughters head. Plus, my daughter had a beautiful head full of hair. My boyfriend said he understood and said he would respect my decision. I went to get groceries one day and came home to my daughter being completely bold. I didn’t think he would, but he has finally cross the line. I was beyond pissed. I yell, I cried, I scream, and I told him I was leaving him. His mom was there too and stepped in and said I was being over dramatic and that she is his baby too and part of their family as well. I told them both I am done with them! Everyone saying I’m ruining my family over something small, but this wasn’t small to me! Like, they shaved my freaking newborn’s head. Im so upset right now and I know I can’t be overreacting! I am wrong or are they???