There is no excuse for this. If he doesn't realize how much it hurt you, it also means that he doesn't realize how much stuttering affects you. A significant other should never try to deliberately hurt you, even in the heat of an argument. What if he had a physical trait that he was very insecure about? You wouldn't make fun of that in an argument. You don't deserve this. No one does. This is not ok. Never has anyone I cared about done this to me. I've even had ex-girlfriends who were terrible to me (cheated, etc.), but even they never made fun of my stutter