Boundaries in poly relationship
I'm new here, and trying to figure out how to deal with my current poly relationship, which has taken some unexpected turns.
In summary: I've been poly before, in a poly-fi triad with a guy and a bi girl. It ended after a year and a half, when the girl became a man... the other guy (call him "Mark") in the relationship is straight. So "Mark" and I decided to be monogamous until recently, and he and I are now engaged.
A couple months ago, Mark brought up the possibility of being with other people. He said he really didn't want me to be with other people (due to jealousy issues), but he wanted to see someone else casually. I wanted to make him happy, and I figured his having a casual relationship with another girl wouldn't come in the way of my needs being met, so I agreed. I also figured he'd come around and let me see someone else eventually... that didn't end up happening, though. It was something of a point of contention between us, but I dropped it when I realized that it just wasn't worth hurting him over.
Mark started seeing a girl right away... literally, two hours after we had the discussion about seeing someone else (Let's call her "Cathy"). He says Cathy told him that she wanted to definitely discuss boundaries with me one-on-one before sleeping with him. But that didn't happen... in fact, I haven't been able to have a single one-on-one conversation with her so far. She's always had Mark ask me if they could do things, or not asked at all (eg, in the case of being intimate at our place). Cathy also said she was interested in me romantically (bi-curious), but said it would have to be on her terms, and that she'd let me know when she wanted to sleep with me. When the three of us have hung out, I've felt like it was hard to get a word in edgewise.
Then last week, I found out from Mark that Cathy is in love with him, and he's "halfway" fallen for her. He says Cathy told him that she could never let someone she loved be with other people, and that it's more natural to be jealous of your partner. She also has apparently been telling him that I'm not good for him, and advising him to leave me. Mark tentatively decided to stay with me after all, and Cathy responded by ending their sexual relationship. He told me that things were over with her, and I was kind of relieved, but I found out yesterday that they're still being sexual online, they still talk every day, they're physically affectionate and he goes to see her a lot. Basically, she's still his primary confidante. He also says that until he's "sure" that he wants to stay with me, he's going to keep dating her... and she knows this, as well.
I really love this guy, and I don't want to lose him. We've been arguing a lot over boundaries, and I know it's hurting him, and he sees me as the sole source of discontent in his life. I'm worried that his seeing Cathy is going to really make it hard for us to have a good relationship. I was hoping that Mark would keep things more casual, or bring in someone we both had chemistry with, and I feel like this is not what I signed up for. I know I need to talk to Cathy, but I'm terrified because I feel like she has me over a barrel... he's quoted her as saying some very critical things about me, and I'm worried she will just tell me to get lost. Would it be possible for you guys to give me some suggestions on what to do?
Thanks for reading...