How are you doing the rules or boundaries to make sure your SO is not cheating or hiding anything in another relationship? I understand everything is about TRUST however it come across as impossible to believe.
I set a boundary with my partners that if they lie to me or deliberately withhold information that affects me or that falls under an agreement we've made, I will end the relationship. I have also agreed to be completely open and honest with them, though none of them have set a similar boundary of leaving me if I'm not honest. However, we also have very specific agreements about what needs to be shared. More on that below.
Phone calls, texting, messaging, dating sites, e-mails and meet ups are quite a few things to keep in check. Are you freely leaving your cellphones/laptops out in the open/unlocked and accessible for your SO to see? Or is privacy still in play?
I have severe trust issues, and grew up in a situation where I was allowed very little privacy. In my marriage to my kids' father (which ended years ago), I was allowed essentially NO privacy. The result is that if my husband even accidentally reads over my shoulder when I'm on the computer, e.g. if he comes up behind me to give me a hug, I flip out on him. It isn't because I have anything to hide; it's because whatever I'm doing on my computer or phone is none of his frigging business unless I *choose* to tell him. And if it involves someone else, I won't choose to tell him a damn thing without their consent, because otherwise I'm invading *their* privacy. At the same time, my husband knows I keep very little to myself, and trusts me to inform him of things he needs to know.
(Like you, my husband is mono. He has no issues with me being poly, because he's seen that he can trust me. And me seeing other people was *his* idea in the first place.)
I mention above that I have specific agreements with my partners about what is shared. With anyone I'm in an actual relationship with, as opposed to just going on a date with, I inform them that I tend to, without thinking, overshare with other partners. (Mostly with my husband and my boyfriend, who I consider co-anchor partners.) I let them know that I will try to be respectful of their privacy, but that if there's anything they specifically don't want shared, they're best off telling me in so many words, "Don't share this." Even with that in place, though, if I have any doubt whatsoever I still ask, "Is it okay for me to share this?" so I'm not violating their privacy.
As far as agreements about what "has to" be shared, it's pretty simple. A first date with someone new. First sexual encounter with someone new (not details, just the fact that it happened), either beforehand if it's been planned, or as soon after it happens as possible, because we consider that a sexual health thing. Anything that directly affects me, for example illness, or a trip out of town that will result in missing a regular date night. (I try to have a set schedule with each partner.) I ask my boyfriend to tell me each time he has a date, aside from prescheduled date nights with other partners, and for that matter each time he's got something happening with friends, but that's because he's my primary support when my mental health goes wonky, and I try to make sure I don't impact his time with anyone else by texting or calling during a date or gaming afternoon or whatever, unless it's an absolute emergency.
I am mono and she is undecided for poly or open. She had my ok to go on a dinner date with someone she met online and to return home afterwards. No sex or anything. It was just to be a first date. Went to work and came home. Asked how it went? It was okay she says. Nothing else. Later in the evening I found a disc in the laptop. It showed a graphic video of her and the guy having sex in our own place for well over two hours. WTF? Confronted her about it and her reasoning for recording the encounter was to make sure she be safe? Odd. I asked where they went to dinner? They didn’t go eat and they went straight for sex in our bedroom as soon as I left our place for work. No protection either even though she is on iud. The rules and boundaries really went out the window here. How do you all do it?
I don't "do it," because I don't involve myself with people who can't keep their word. If she specifically agreed to "just a date at a restaurant, then home, no sex," and then didn't even go to the restaurant and instead brought the guy home to fuck, she has broken her word, compounded by endangering her sexual health and yours. That isn't how poly is meant to work. Poly works by clear, explicit communication, honesty, and keeping agreements or renegotiating them rather than just ignoring them.