Did you also have a rule that you and your bf could not have sex alone behind your gf's back? And did you ask your gf what sex acts she didn't want your bf doing with you before forbidding him from going down on her?
If not, then there was an unfair imbalance (and people always want what they cannot have!) and your rules treated her as less important. I'd feel like shit if a couple treated me that way. Besides the fact that the rules were pretty unrealistic.
I haven't read the entire thread in detail, but I do agree that you don't need to walk away from this. It is clearly time to renegotiate boundaries. But I hope you do so with the thought process that a triad is not a Couple Plus One situation. It is a One Plus One Plus One situation, and anyone having a place above another just plain sucks and is disrespectful, if you want it to work as a whole.
There are four relationships in a Triad:
A + B
A + C
B + C
A + B + C
Treat them all as equally important, nurture each according to what is needed by those individuals, and let each relationship grow of its own accord. Sit down regularly to check in with each other. Have boundaries that are about what you need to be happy and satisfied, NOT about how to control other people's actions. You have already found out you can't control them, anyway. Who cares that you were with him first, you're with them both now. Don't treat her as second class. Loosen up, lighten up, and move forward with a more generous spirit, and I think the three of you can heal and have something really great together.