Without hearing the other side(s) to this story, I can suspect the following:
It sounds like your idea of "open/poly relationship" and her idea of "open/poly relationship" are not the same thing. Unlike the other responses here, I don't see egregious disrespect or "cheating", I see two people in a relationship who are not on the same page. When you discussed this and "let it marinate", did you clearly establish certain things such as:
1) what's the difference between a "friendly" date and a "romantic" date, and at what point does it become the other partner's business when a "friend" becomes "more than a friend" or a "friend with benefits"? Personally, when I go out with someone who is "just a friend", I don't call it a "date", and when I go on a "date", there is the implication that there could be sex involved AT SOME POINT in the future.
2) things do not always appear the same on paper or in conversation as they do in practice. You can "discuss" "opening a relationship" till the cows come home, but you will never know how it really affects you until one of you finds someone else to be with and the other has to deal with it as a reality.
3) having said 1 and 2, it is up to the partner who finds someone else FIRST to make sure that the "solo" partner is told about it and brought up to speed as the previous discussion(s) established. However, it is BOTH partners' responsibility to each of their selves to say something to the other when something needs to be said. Just because you live with someone or are in a relationship with them for a long time does not make either one of you a mind-reader.
I don't see any disasters-in-the-making here from what was said in the OP. I see some typical mistakes and counterproductive assumptions were made by some people who are new to a certain way of life. My advice to both of you is to think about it, write down what your hopes, fears and expectations are, and especially try to identify what is real and what exists only inside of your mind. The last thing is something that I struggle with and it helps to remind oneself that just because you get along with someone really well, it doesn't mean their mind operates the same way yours does. I see people on this forum often discussing the difference btwn sympathy and empathy. You really do need both, because sympathy tells you how YOU would feel in someone ELSE'S situation, while empathy is about understanding how someone ELSE feels in ANY situation.