Your boyfriend was pretty much being a prick
It didn't sound like he promised you anything, so he might technically need to stick with his word, but things change sometimes. I would be more upset that the dickhead doesn't realize that "Don't worry, I don't need to do that thing with her tonight." make give you the impression that you needn't worry, he isn't going to be doing that.
So after he did, you would think he understand why you might be upset, and realize his part in it, I don't know, maybe said, you're right that wasn't cool for me to say that and then do the opposite, but it is ultimately me choice and not yours so I took the liberty to do so.
That's what I would expect from a guy who is responsible, capable of owning his own shit, even though he definitely sounds like he has a prickish attitude of I really don't care.
Whatever agreements you guys make with each other is totally fine, I haven't read through the comments but I seriously doubt anybody is asshole enough to tell you and boyfriend what you can and cannot or should not do unless they are involved with you.
But if anybody was going to that, an online poly forum would probably be the most likely place as these online forums have a lot of great people, but part of being part of an online poly community is allowing for a minimum of 3 fundamental principles that make up your poly core that it's cool to deny for aspects that aren't in your core, we all do it, some people don't like being called on it.
Speaking of called on it, anonymous on the internet? you mean the computer one BG? Everything is traceable, in fact a friend of mine who also has a good job at a junior college, but he got the job because of his dad being a prominent name in department he works in, and when he over heard me asking how to trace intrusions on my piddly network and this random person gave me a completely fabricated answer. Luckily my friend waited for him to leave before pulling me aside and saying "he knows damn well there's nothing anonymous about the majority of users on the web, that was one of the fuckers who used to stream you live after highjacking your web cam" and I was like nawwww, he wouldn't do that over being a myspace friend of a girl he liked, and he proceeded to tell me to keep playing dumb and he'd shoe me how to find out for sure, so long as they kept doing it. But it's a long story and even just to get into the tip of the ice berg would be fair,
anyway charmed, you right well, almost familiar but at four posts it couldn't be familiar from here. I hope you stick around