Hey Chloe, I appreciate the in-depth responses and you challenging us back, I find this sort of conversation to be really interesting.
I can speak to my own post above (and obviously I agreed with Nyc's, though I chose different areas to focus on). I started off by talking about your husband's transgressions and motives very deliberately because I felt like that was the most immediate cause for concern. I completely, completely agree with you that no one should be coerced into compromising their boundaries based solely on someone else's desires, and that shifting of boundaries should happen via conversation and agreement, not experimentation and pushing. Him doing that was not ok, plain and simple.
I went on from there to discuss why I thought the boundaries were problematic and yes, there is some value-judgment there -- not on you, but on the boundaries themselves.
In your post in the Intro section you refer to your third as your shared "girlfriend". If you're not opening up the emotional side off your relationship, then I find this word choice completely inappropriate. The word implies emotional involvement, I would think, which can easily develop from repeated shared sex alone. And indeed you're not just having sex you're doing romantic things together. But love is not supposed to develop? I don't see that as a realistic plan. It's possible that it'll work that way, but equally possible that one or more of you will develop feelings you didn't mean to have. I see the word girlfriend, I see sex and romance and friendship and then I see "but no feelings, k?" and words like "robot" and "hooker" begin to seem not so strange, because unlike another lover or partner, you could rationally expect a robot or a hooker to not fall in love.
We see story after story here of people who are in crisis for this exact reason -- they entered a situation that was very conducive to developing emotional attachments, like the one you've set up here, and then everything went into meltdown mode when those attachments surfaced. No one wants to see that again. I am not in favor of boundaries meant to prevent emotional attachment because a) I don't think the goal behind them is smart (inasmuch as it defies basic understandings of human nature and emotion), healthy (inasmuch as it often leads to people doing their damnedest to deny their feelings and suffering as a result), or realistic (inasmuch as it doesn't seem to work), and b) because over time they seem to be damaging to people's self-esteem.
I know you say your third is a busy person and this is her fantasy-time, and I can see how her subby desires could make this work all the better for her, but personally, sub or no, if I had a "boyfriend" and I wasn't allowed to ever just say "hey" to him, I think I'd start to feel like shit. Or if I looked into his eyes during sex and realized I was falling for him and knew that was against the rules, I'd feel like shit. When an adult is in a relationship with another adult, I believe that it is only fair and humane that they should get to bond in their own way on their own time to some degree. This does not mean anything goes and that it should be ok for them to move in together or disrespect you or anything like that. And there may well be nothing there! But if there's not, *why* keep an enforced wall between them with you as the gatekeeper for communication and physicality? So that nothing ever does develop? Well, see above for my thoughts on that.
You say the fear behind your boundaries is that your husband won't be worthy of your trust, that he'd be willing to be dishonest with you. But as you've seen by his behavior, rules alone don't make someone act in a trustworthy way. I truly don't see how a fear that someone is a rule-breaker can be solved with rules. The rules can demonstrate whether or not he *is* worthy if trust, I suppose, and indeed he's been showing a worrisome track record there. I'm really glad you two talked and that he seems to understand where his actions were coming from. But you wouldn't say "I'm worried I can't trust you, so to assuage my fears we'll put a couple boxes around the house that you're not allowed to open." That would be absurd. Rules exist for their own reasons, and that's why I've focused above so much on the no-emotional-attachment thing, because that seems more pertinent to me.
I respect your experience in poly and your ability to say what you feel is right for you. It took guts to step out of that bathroom and say "stop, we agreed you wouldn't do this" and you were right, absolutely, because again it was NOT cool for him to casually break his agreements with you. But I don't feel we've gotten to the bottom of what's going on here -- *why* are you guys not open to the formation of emotional attachments? What do you think would happen if one or more of the three of you fell in love? Why would your heart and soul be so hurt if someone did the natural thing and got emotional?
This is polyamory.com, not polyfuckery.com or swingers.net, so is it really so surprising that your audience here would balk at rules meant to prevent loving attachment? We're all about the love here.
For context, perhaps it would help us to know what your husband's boundaries are? Does your third have any stated boundaries of her own?