My partner and I have agreed that we’re allowed to have sexual encounters with other people from the start of our six month relationship, but have never been intimate with anyone else without the other being present. While I was out of town, he slept with a woman we were entertaining playing with together. He didn’t tell me about it until yesterday, almost a week from the event. He said that he didn’t want to ruin my holiday and last night was the first opportunity when he felt we had a chance to talk. We didn’t discuss when we would disclose encounters to each other, although I did state that I preferred he told me beforehand. That’s alright, that was a learning moment. However, what’s really bothering me is that he said that he wouldn’t sleep with her by himself and that the three of us would play together first. Additionally, I feel hurt that we were intimate before he finally told me that he has slept with her. I’m simply hurt/confused... This is my first foray into open relationships, and my boyfriend’s first intimate encounter outside the relationship. Am I overreacting because I’m jealous or so I have a valid bone to pick?
You've got a false dichotomy in your question there. When a couple is in conflict, remember that it's you two vs. the problem, not you vs. each other. Your feelings are totally valid, but that doesn't necessarily mean he did anything wrong.
Stuff like this happens all the time - you set up rules and expectations to protect yourself, but life has a funny way of getting around those. Events unfold, energy between two people sparks along its course, and things play out differently than how the ideal would be. Your partner was probably very startled and frustrated by the whole thing, and anxious about telling you.
You're right that this is a learning moment. You two are learning to be more clear with yourselves and each other about what, exactly, you need and want out of this arrangement. You're also learning that the real world might not play nicely with your clearly defined boundaries, and that you'll need to find areas of flexibility and compromise. Figure out what's a need vs. a desire, what might not be realistic, and where you can give each other and the world some 'wiggle room.' Remember that the more rigid and specific your rules are, the easier they are to break, which feels like a betrayal. But, if they're too wishy-washy, then you might accidentally hurt the other person without realizing that they were more serious about something than you thought.
Keep talking this out! Remember that feeling hurt is totally okay, but it does not mean that the other person is in the wrong. This is a sticky, feeling-laden thing you're doing, so feelings are going to happen. Work through them and with each other.