Step parents welcome at school events?

I drive my step daughter to school every morning after her mom drops her off to me, I help her with homework when she's at my house after school, I take care of whatever she need when she stays at our house. Today for example she has an art show at school (her teacher said "the more the merrier") so I told my fiancé I'd like to go today in which he responded by asking me if I asked her mother for permission to go. He said she is planning on her mother and aunt to so it's rude for me to just show up with him. I told him I would think it would be a given that as his fiancé where he is welcome I am welcome. Also regarding his daughters school where I am involved regularly. I don't think I should have to ask her for permission, it is his daughter too and I am his fiancé. Am I wrong or is he?