I guess a little more background is in order.
I'm a student in university (in my last year), I work part-time, and I have disabilities. When the conversation about poly came up, I had just gone through a really stressful week involving summer class finals and having a chronic health condition kick in. Mark's work had been really stressing him out, but I asked him to help me get to the doctor because my vision was messed up (he telecommutes, so has flexible hours), and he agreed. Anyway, we'd both been under a fair bit of stress, and on the way home he was talking about how he felt like I was taking too much out of him, and my problems were stressing him out. So I asked what I could do, and one of the things he said was, "let me see other people." I totally didn't expect this, given that he'd been the one pushing for us to be monogamous after our triad broke up, but I figured it was worth a shot. After all, I'm really busy, and although I spend most of my spare time with him, there were ample times when I was away that he could see someone else. I was cheerful about the idea, encouraging, figuring that he'd be nervous about my response. I assumed (bad, I know) that we'd have more conversations before anything happened, but a couple hours later, he brought up a girl I'd met a couple times (Cathy). He sent us both an email asking for permission to do things sexually with her, and I sent them both a reply saying, "Have fun!" The next day, they ended up in bed together. It was way faster than I expected, but I figured that if he wanted to do this stuff again so much, that would explain why he'd been so stressed out...
Through it all, Mark had some misgivings, mostly that he was hurting me by doing this when I couldn't. I talked to him frankly about whether it was what he really wanted, and it really sounded like it was, and the only barrier in his mind was when he worried about my feelings. I honestly wanted to see other people if he was, and told him this, but he straight-up said that he would just leave if I did. He talked about how he saw his being with other people as a failing, and that doing it again was making him realize that it just wasn't worth it, and that he'd pretty much gotten it out of his system. He said that "Cathy" would be the last time he was poly, and that after things ended with her (she's been looking for a monogamous partner), he wanted to be monogamous with me. But he said that he was worried that I'd always want to be with other people, and that I'd never be satisfied with just him. He also started saying that he hadn't really wanted to be poly that much, and that in fact I had "pushed" him into it by encouraging him and suggesting that maybe this was what he really needed.
Right now, he's told me that what he really wants is to just be with me, but that Cathy "makes him happy", whereas he and I have issues still standing (at this point, mostly about her). I don't see how you can compare a two-month relationship to one that's lasted for years... it's impossible to have emotional baggage in such a new relationship, isn't it? It sounds a lot like Mark pours his heart out to Cathy when they talk, and that he's emotionally dependent on their relationship, which would explain why he was clearly so torn up at the idea of not talking to her anymore. I really think that he wants to make things work with me, but as long as Cathy is acting in a therapist role, it's going to be hard. I told him that I wanted them to stop talking about sex, stop seeing each other alone, not discuss me, and just generally limit their interactions for a little while until things had cooled off. But he said no, they're still close friends, and if I set these boundaries then he'd tell her that the reason they couldn't be at her place, or whatnot, was because I'd said so. I didn't want to give Cathy more reasons to criticize me, so I backed way off and asked him what he thought I should do. He said I should just trust him. I'm trying...
I know we need to set up some appropriate boundaries, but I'm not sure what they are. Do you guys think my requests were unreasonable? The petty side of me wants to see their chat logs and emails, but I know that would be way over the line, and I want to respect their friendship.