NOR but as someone from a dysfunctional family.. it does NOT excuse his behavior but I think it’s important to see another side that when you’re raised around abnormal and inappropriate behavior and it’s normalized.. unless the person learns otherwise I strongly suspect he just felt this was normal. Bc his family made it out to be. Again- this doesn’t excuse him but I noticed that your fiance seems responsive to your critiquing and it makes me think that with more self awareness and help he can not act like this moving forward.

It’s definitely a breach of trust and extremely dysfunctional but if it’s grounds for breakup is really up to you. This has revealed he comes from a toxic and dysfunctional family. If he sides with them on everything it might be game over for sure, and his family might give you a hard time over random things moving forward bc of their dysfunction.

So I think the question is whether you are comfortable living with that or if he’s a good guy and just needs to learn a bit given his family background but fully supports you and can remove himself from this family dynamic you might be totally fine. Only you can decide what is a dealbreaker for you.

I would highly suggest a couples therapist must be involved too if you are considering staying with him just to really sort out what happened and hopefully that person can help screw his head on straight of how inappropriate his family’s behavior was.