You are not over-reacting. You had agreements in place, you had communicated with him your need to know about anything that came up asap.
"Planning on" telling you? Worried about how you would react around them because you were working on a project together? Perhaps he should have thought about that before he slept with them! And why is he worrying about how you would react around them when he should be worrying about his relationship with you!
I, too, would be completely blown away by his actions - they are unconscionable. Even one "slip up" - getting carried away with a good friend one night at a party, for instance - could be a deal breaker for many people (mono or poly). But seven - seven is no "Ooops, honey, I screwed up." Seven is systematically taking advantage of the situation with no regard for you, your friendships, or your relationship.
To be successful, poly requires honesty and trust - and even then there is no guarantee that things will work. He failed on the honesty and destroyed your trust.
"Poly" is not an excuse for being a FLAMING ASSHAT!
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe