I can understand how you might find the welcome here pretty intense. I hope you can still find a peaceful space for yourself here.
I for one would like to read how things develop. In the blogs section there is generally less commenting/advicing going on, so if it gets too heavy you might want to move there.
To the original issue you posted, I totally agree that your husband violated the agreements you had, and is completely responsible. I'm happy about all the communication there seems to be going on, and it seems like you got to the bottom of it.
It sounds like your boundaries you have work for this situation, since none of you have expressed a desire for more one-on-one time/sex. (And while some here have suggested that might be behind your husbands actions, I totally agree with you that if it is, he needs to communicate that to you.)
Since you say you would be open to different agreements later with your girlfriend or in some other kind of situation, I'll write a thought I had, even though it's not very relevant at the current situation.
From what I gather, you have reasons for your boundaries mostly in your husband's tendencies of having a poor judgement when he is in NRE. It seems that when he is in NRE, you can't really trust him to take your feelings into account very well. Am I understanding this right?
By the way, I totally understand why you would have a hard time trusting his judgement given the choices he's made in your current situation.
It seems that your boundaries (no contact between them, no sex with girlfriend without all three of you there) are in place to avoid those issues. And I think that's fine for now: there is nothing wrong with enjoying what you have, and it seems to be enjoyable to you all as things are. However, if, at some point and with somebody, you decide to move forward, I'm wondering how can your husband "prove you wrong", i.e. show you that he can be trusted? With your current boundaries, if he sticks to them (he hasn't, but say from now on) the issue of poor judgement is avoided altogether. Not violating the boundaries you have shows you nothing, except that he is willing to respect your feelings. Which is obviously important. But I'm wondering if there would be some agreement, which gives him a little more responsibility. That way, if he keeps to that agreement, you see that he is working to respect your agreements, and you might begin to trust him more. Then again, if he doesn't, it may tell you that you really can't trust him. (I'm all for second chances, but I would also draw that conclusion if he violates the current boundaries again.)