I found myself thinking about this situation some more today. It's funny how one person's very specific, personal story can end up being a sort of stand-in for everyone's thoughts on an issue or a range of issues. This situation definitely pings some of my own issues/questions/feelings about my relationship with my gf and her husband. Before I go on, note that at this point I am not trying to say anything whatsoever about whether your husband acted appropriately or whether it's incumbent upon any of you (or even advisable) to change your boundaries at this point, I think I've covered my thoughts on that thoroughly.
One thing that I find a little confusing is that your husband is saying he's not interested in one-on-one sex with the gf. If not, then why did he choose (whether he was consciously "choosing" or just acting) that particular boundary to push multiple times? Maybe he doesn't feel like it's something he at all needs right now, but to say he doesn't want it seems to contradict his actions. Then again, I suppose people sometimes do things, like lie, that don't represent what they really want to be doing per se, and are instead just manifestations of something else that's going on.
But if he (and you for that matter) have an emotional, growing relationship with her, why *wouldn't* you want at least the potential for one-on-one sex? I find that threesome sex is very intense and definitely creates intimacy, but that couple sex holds (for me, at least) a different, perhaps more concretely bonding, sort of intimacy. It's hard for me not to see an agreement that says "no couple sex" as not being about preventing that sort of bonding. Otherwise, if it's truly just that nobody feels like it right now, why have an agreement about it at all? After all, if nobody wanted bdsm, you wouldn't articulate an agreement that says "no bdsm", most likely. I get that it's about trust issues with the hubs more than anything else, and I think that's definitely valid, just pondering.
Also, I don't think everything has to be perfectly balanced by any means, especially not at this stage -- after all, you and your husband have a long-standing primary partnership and you're just getting to know this girl -- but I do see a question of balance here that I find intriguing. If you're building a threeway sexual and love relationship but you and the husband can have couple sex (I certainly assume this is the case) but neither of you can with the gf, that puts her in a very different role, one that I find to be very intense but less personal, less partner-y.
It's not just about sex, of course, it's about alone time generally. I imagine there are things you've shared with your husband when it's been just the two of you that you might not have wanted to open up about under any other circumstances, and vice versa. I've shared some very personal things while with my gf and her husband, but there are other things, little personal revelations that I couldn't even have predicted, that only came out when I was alone with her. I truly believe that humans just act differently in dyad situations in ways that are both revealing and, again, bonding. I think this is why I see this particular agreement as fine for starting out, but as problematic if it should stay in place and try to co-exist with the goal of one or more serious love relationships.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.