Welcome to SI.
I know that every relationship has different dynamics, many of which are agreed upon, some which unfortunately aren't.
To me, and open relationship means that boundaries are anything but firm for fidelity. Yes, you had a spoken agreement, but because the floodgates were open to experiencing ANYTHING with another partner, it's easy to imagine that anything resembling an allowable EA might give way to PA, and vice versa. We talk about the slippery slope here... I think your relationship was greased from the start.
Yes, he lied and you were betrayed. The thing is, he was a kid in the candy store and there were no parents around...
I had a relationship like this in college. I told him I was ok if he wanted to date other women, just to please let me know so I was aware of it. He kept me on the line and had girls on the side he hid from me. I couldn't fathom why at the time, but later I realized it was because there is an inherent guilt that comes along with stepping outside of a relationship, even when there's an agreement that it's "ok."
Bottom line, he broke a commitment to you and you get to feel what you need to feel and do what you need to do. As for the confusion... I think that either new, more exclusive boundaries need to be discussed or he is just going to continue to do whatever he wants and hide behind the loose constructs of an open relationship.