Hi all, this post is almost my cry for help. I (M) am the monogamous of the two, and my partner (F) has recently asked me if she can start seeing a coworker of hers. This would be the first partner she will have since we've been together and I am struggling to come to terms with it. On paper it seems so easy, "what does it matter, knowing that she comes home to me every day". But now my feelings are a mess and I am struggling to sleep.

Initially, I tried to say no to the idea of her dating him. This was because I didn't like him as a person. As far as he knew, my girlfriend and I are planning to marry (we are). And yet in spite of this, he's actively shown incredible interest in her eventhough he knows of my existence. That just feels hurtful and disrespectful so I tried saying no. However.. I was still open to the idea, albeit unwillingly. Because at the end of the day, I am monogamous, my partner is not. But I would like to be respectful and supportive of her. So the door is still open for discussion. I want nothing more than to be able to talk about this and come to a compromise where we both feel like our boundaries can be respected.

But this didn't go over well with my partner. And it got really messy. She feels like she is being repressed, and that my reasons for not wanting her to see him are too restrictive and that I am too controlling of her life and choice in partners. Our compatibility came into question and basically so did our entire relationship, which really crushed me. To my understanding we were 100% committed to each other no matter what, and so that to me meant that we would work things out. But I was basically given an ultimatuum of "either be okay with this coworker happening, or let go". That hurt a lot.

At the end of the day though, I want to make it work. The two of us as individuals are perfect for each other. We really are. The only wedge right now is my monogamy and her polygamy and finding a good middle ground that works for both of us. And so my questions to you lovely people:

1. What do I do?? Where do I begin? How do I work through this and come to terms with my own insecurities and grow from it to become a better partner?
2. As hurtful as this sounds, and I feel like an asshole just saying this but I feel like I must.. Am I disrespecting my own boundaries? I was honestly shocked when I was given the ultimatuum and hurt to my core. But I don't see this as being something we would end things over.
3. What sort of boundaries can we talk about? What worked for any of you? The poly world is completely unknown to me and so I will take any sort of advice. Are there restrictions that I can put on things like sex? Wearing a condom?
4. How do I come to terms with splitting my time up with her with someone else? Does that get easier? Would it be better for me to meet this guy and get to know him??

I'm sorry it's a lot. If I sound like a complete douchebag from this story please feel free to call me out. From where I'm standing, I don't particularly feel heard by my partner right now and there's a disconnect in our communication where I want to find middle ground that works for both of us, and it feels as if she's almost forcing her decision onto me. I don't feel like my boundaries are even being noticed, that I am beyond my comfort zone. But I love this woman with all my heart and I want to make it work if I can. So any advice is welcome, even criticism. Love you all.