My (32f) fiancée, Mark, (35f)of 5 years and I decided to open our relationship 6 months ago. When we met, he was the one to introduce the concept of poly to me and said, though he was happy to be monogamous for now, he thought it was important that we eventually considered opening our relationship in some way in the future. We’ve had innumerable talks about it over the years and he made a few comments about wanting to get started himself which I agreed I was theoretically open to, but almost every time got emotional at the thought of it happening now.

About a year ago I started a new job and became instantly attracted to a coworker (27m), we’ll call him Lee. The first 6 months of that were simple, exciting, flirty (fiancée and I have always been okay with more flirtation than more traditional couples). Nothing ever crossed the line and I never knew if Lee felt similarly or if he was just mirroring my behavior and having fun. I also didn’t shy away from telling Mark about my feelings, but since I wasn’t sure if it was something I was ready to pursue, Mark didn’t realize how strongly I was feeling about Lee.

6 months ago, I asked if it was something I could pursue. Mark was onboard immediately, we ordered More Than One, I kissed and talked to Lee that weekend, and things went off from there.

A few things happened since Lee and I started dating:

1) since we work together, most of our time is spent after work together. Mark & I, just prior to this didn’t share a similar work schedule and weren’t spending much time in those hours, but at the same time as the start of my relationship with Lee, that shifted and I think he expected to go back to after-work quality time together though this was never explicitly stated

2) Lee and I never made date plans on nights that Mark was available. We went on proper dates about once a month, and though I’d stay out with him for 6 hours, it was always on nights that the bulk of those hours weren’t interfering with time with Mark.

3) I kissed another person during this time that crossed a few boundaries for Mark, including some we had discussed. It was not a romantic thing, but I’d been drinking. Lee was also around.

4) Mark expressed he wasn’t feeling prioritized as much as he needed to be in the latter 3 months of this. We had many difficult talks but he insisted it wasn’t Lee or even necessarily the poly that was causing this. But a pattern of behavior of mine that was being highlighted by our new situation.

5) Mark never dated anyone in this time. We both downloaded apps and he tried, but I think it didn’t happen for a handful of reasons.

6) The final straw. Mark was out of town and Lee and I had a big night out, came to my apartment to walk the dog (which is off limits for sexual activity), and spent the night at his place. I accidentally left my phone at mine. In the morning, once I was home, Mark called me asking how my night was. I told him everything and he revealed to me that he thought I had broken some very strong boundaries (he used term cheated). Mark had seen on our camera that we came in, I turned the camera around, and then never put it back so it looks like we never left. I explained the night in more detail, and I could tell and he confirmed he believed me once I explained the story. Reasoning behind turning the camera is that it alerts us both with motion sensor and also records audio. I found that all really inappropriate while I’m having a candid hang with Lee and also didn’t want to send Mark a million notifications in the middle of the night. I provided proof that we weren’t at our place that night and Mark confirmed he believed me but had spent 5 hours in this state of panic and convinced I’d done something unforgivable.

So now, Mark’s expressed that he doesn’t think we can get past the issues of prioritization and respect nor can he trust me to make the right decisions for our primary relationship while I’m with Lee and has handed me an ultimatum (his words). Trust has never been an issue for him in the past.

Can anyone offer any advice on this situation or how to express everything to Lee without overloading him?

TIA. I’m absolutely heartbroken.