Hi all, hoping to get a lottt of advice here. A bit of background, I (M) have always been monogamous in all my relationships and have always felt like I needed a romantic connection to someone before I could sleep with them. Partner (F) has always had more open relationships. We're both committed to one another and plan to marry. To compromise for each other, I told her I would be open to the idea of her seeing other people before we got married. It was her idea and desire to put the open life behind her after getting married. For a while she wasn't seeing anyone, told me that I was fulfilling everything she needed and wanted. This made me feel very comfortable. But now at her new job she has found a guy who has been hitting on her since day 1, eventhough he knows of my existence and does not yet know she's open to the idea. So already I honestly don't like this person very much. She initially tried to dissuade him but after a few months she's feeling charmed and wants my permission to be open with him.

But I thought this would be a lot easier for me than it is. I knew going into this relationship that this would happen and have tried to mentally prepare myself. On paper it seems so easy, "so what if she sleeps with someone else? She comes back to me anyway." But now, the insecure intrusive thoughts are destroying my mental health and when I'm with my partner I feel like I'm even being distant at times. We're still talking about it a lot. Her ideal is that she would like to see him, maybe once a week. My ideal is that I don't like this guy and I would like to say no. But I also don't want to disappoint her so I'm thinking of letting it be a one time thing. I have so many questions.. If not him, then who? How many rules and boundaries am I allowed to put in place so that we can both be happy about this? As far as I know, she's happy I'm trying to let it happen. She is respecting my decision and told me that if I'm so not okay with it, it doesn't have to happen. But she has also told me that she would feel repressed if I denied it altogether. However, it isn't helping that she texts him a lot while we're together, and its quite clear how eager she is to sleep with him meanwhile I'm struggling with even the thought of them spending time together, let alone sleep together.

She has been very good about reassuring me, showing me affection, and reminding me how committed she is to me. But honestly, I'm slightly below average in height (5'7), 6/10 looks, 7 at best, while he's tall and is a good 9/10 in looks for her. So my own lack of self confidence is getting in my way, I feel like I'm inadequate. I hate the idea of having to split up what little time I have with her, so she can go sleep with a coworker on a regular basis. So I have a lot of questions. Because I want to respect her openness but also have my boundaries respected.

1. How do I come to terms with all of this? It came up last week and we've talk about it a lot but honestly it feels like its just in circles where I tell her my insecurities and she reassures me that at the end of the day, I am the one for her and this guy is just sex. I feel like I need something more concrete but I don't know how to get it.
2. How many boundaries/rules is too much? I don't like it when she's texting him while she's with me. She isn't a fan of condoms during sex because she has an IUD and I'm expecting she won't use one with him either, but I'm not keen on the idea of no protection and ejaculating inside. I don't know how she'll respond to this.
3. Like I said earlier, I actually don't like this guy very much. I don't think it's okay how much he reaches out to her and flirts with her while knowing she's in a relationship, without knowing she's open to the idea. Is this normal? Is this a criteria where I am allowed to say no? How damaging would it be to say no to him? This would be her first partner in our relationship. She's reassured me multiple times that she would tell it to him straight, tell him the boundaries they would be allowed to have, and if he can't respect it, no dice. But I still dislike him.

I know I'm not taking it very well, but I am trying my best. I think that counts for something. But also, if I say no here, then who will I approve? I don't know either. Also, I think its important to include the fact that when we talked about our compromise of me being open to the idea of her wanting an open relationship before marriage, there was a misunderstanding. I thought she meant she wanted to do hookups, one night stands, etc. Because she has a body count shes trying to hit. So I said okay fine, if you want to hit that body count, I will try my best to be okay with it. Her understanding however, was that I would be okay with her having regular partners. So there was some miscommunication which is now causing me quite some grief over what I am okay with and what I am not okay with.

If you've read this far, thank you. Again, this is my first experience ever. The concept of ENM and poly as a whole is foreign to me. I've never felt like I needed it so I don't personally understand it. But I want to make my partner happy so here I am asking for advice! I'll take anything. Even criticism. I don't know if I'm being overbearing or childish or not respecting my own boundaries enough. Most of this I've discussed with my partner, some I have not. Once again, thank you.