Hey there, welcome.
You are seriously doubting yourself here? I can't really see any reason why you should feel like you are overreacting right now. I would be through the roof. It doesn't matter what name you give this or he is trying to excuse his behaviour with ... he completely broke and violated your trust, as well as your 'friends' (I would seriously reconsider if I would call those people friends any longer as well). What is this situation?! Poly doesn't mean that you have an excuse for sleeping around as long as you plan to tell your partner at some uncertain point about what you are doing. Even one friend would be a total deal breaker for me as well as one stranger, friend makes it even worse from my point of view, let alone a whole bunch of them.
I can only tell what I would do: I would distance myself from those people and him. No one who is able to go behind my back and look me in the face afterwards without feeling the need to tell me what happened is someone I would want in my life in any thinkable way.
Sorry that this happened.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.