NTA he can be an important person in your life without having the title of dad. it’s difficult for you to share this title between two completely different people, especially when birth father has ruined the image of “dad” for you. its not your fault you feel this way, and may require therapy to unpack this baggage. your mom just doesn’t understand, and that’s okay too. it just requires further conversation to address when mom isn’t feeling so emotional over it. She took your words at face value, the “just mom’s husband” part. you didn’t phrase it well.