I am sorry that you heard hurtful misgendering words from your partner.
Disclaimer: i am genderfluid & hang out with mostly radical non-normative queer folks. So i can only speak from my own experiences, personally & socially.
Tough to deal with. When a cis/normative person (or person who has always assumed they are cis/normative) dates a trans person for the first time, chances are they will start thinking about gender a lot. They may question their own sexual orientation, question social assumptions around gender, question what "makes" a person a girl or a guy. And well, that means they might say some pretty inappropriate things. They might use misgendering words because more inclusive lingo like "assigned -- at birth," etc, doesn't come naturally to them. They might not realize that their partner isn't necessarily the person with whom they should process their feelings, or expect to answer all of their "trans 101" questions. They might not know quite how to say that which is the truth: "this is a new experience for me."
Sometimes - not every time but happily it really does happen - this process may start with stupid comments, and end with a person becoming an amazing lover, friend, and ally.
Granted, Other times, the hurtful words really are just - well, some people are jerks. None of us has to sacrifice our own well-being to play teacher for someone who won't make a decent effort to learn and to be kind.
How to respond is your choice and whatever you choose, if it's right for you, it's 100% valid.
Hugs to both Trillium & Kelly.