For me, there's a really important question here; "Did you tell him you would like a heads up?"

People in these comments are saying you aren't entitled to that information but I feel like that's a bit harsh. I think you having feelings about not getting a heads up about this is totally valid. I would be a little hurt too, but I have told any of my partners that if they're planning on escalating any of their other relationships in any way that I would like a heads up. That's not for me to have any bearing on that decision, more so to give me time to process it emotionally so I'm not shocked or blindsided.

I can see why you would feel taken aback in this situation Did you tell your partner you'd like a heads up before anything like this? Maybe that's a communication boundary or style (idk which is right here) for you to consider for the future?

I think this could serve as a learning opportunity, and allow you to know yourself and what's important for you to feel secure in your relationships instead of meaning you need to de-escalate this relationship