How is he responding to the fact that you're hurt and upset?
Does he understand that he broke a boundary? Does he try to understand why it's so upsetting for you?
My husband broke a major boundary (with his GF, who I don't like, so - similar situation).
I could not for the life of me understand why he did that. My kind, considerate, caring husband. Turns out that even though we had specifically talked about the boundary, he still managed to interpret it in such a way, that he did not feel that he did anything wrong when he broke it. Which was SO frustrating. It took many many hours of conversation before he finally GOT it, that the boundary meant something different to me than it meant to him, and only then could he truly apologize and could I truly accept his apology.
I guess what I'm saying is that even though you say things like "i made this clear to him" and "he knows this, we talked about it" it could still be that you don't have the exact same view about the rules and boundaries. I found this out the hard way! With new boundaries, I am MUCH more cautious, I explain what I want and why I want it and I think of all possible caveats, and I make my husband repeat what I said which seems incredibly childish but you would not believe how many times someone will repeat what you said and repeat it wrong... because people will hear what they want to hear, especially when they want to have sex.
so, while it's ok to think he's a fucking monster douchebag for now, while you're hurting, he is also just a human being who messed up. It all depends on how he is willing to deal with the messing up, and if he's willing to trust YOU more (because the secrecy about meeting or not meeting her, feels like lack of trust to me)
Good luck, it's not a very nice situation, I know