Quick background. Partner cheated on me for 2 months with the mother of one of our children's classmates. We've tried to work past this, he's been sorry, told me that it's his fault, a mistake, etc etc.
Realised today that another mother from our child's class has messaged him, inviting our child to a party. He barely knows her. Something didn't sit right with me and I made him show me the messages (he initially hadn't told me about them). I saw he thanked her for the invite. Initiated small talk, then randomly apologised that he couldn't accept her friend request right now as we were having problems in our relationship and I would be angry. She tried to rebuff this and say that she'd only requested him so he saw her message and he added 'I know, but Switchit doesn't see it like that'. Cue further 10 messages of small talk. I've hit the roof over him sharing personal information about our relationship and covering it up and he's adamant that's he was being polite and doing nothing wrong. Am I being a mug? Or am I overreacting?