AITA for reacting the way I did with my bf joking about the n slur

Here we are, sitting down after watching a movie and eating dinner and I decided to text back my cousin. Im (f21) sitting next to my bf (m21) on our couch while drafting the message and he asks if he could write something on my phone real quick. He said it would just be 3 letters. I said okay sure thinking he was just gonna put something stupid. I look over and he is typing out the first 3 letters to the n slur- and freak out. I grab my phone frantically from him, hitting my plate and spilling leftovers on the floor. I ask him "baby, what are you doing?! Baby what are you writing?! Why would you write that baby?!". I angrily/upsetly (?) pick up the pieces of food off the floor and stomp over and use the vacuum. I dont think he was expecting that reaction but he just goes to our bedroom and puts the pillow and blankets over his head. I sit on the end of the bed and now calm, ask him what that was all about and I know he thought he was being funny but its not okay. I kept saying I think we should talk about it and he just said "he doesn't want to talk about it". So now im giving him his space, but im still kinda shocked. He and I are pretty liberal and dont take light to those things but like idk, internet meme culture ruins brains? I truly dont know if im overreacting idk