It certainly seems there was some misunderstanding about what you as a couple were expecting of this other couple. Was there actually an agreement to be monogamous to each other (as a couple), or did they agree to not look for "new" couples, but reserved the right to continue to play with their other couple? This seems to be the crux of the misunderstanding. If it was the former, I can understand your husband being angry, because he felt a promise had been broken. That said, he can't force a couple to do anything, and his continued anger about the subject is only going to make the situation awkward and challenging.
Maybe when you meet again you can have an honest and frank discussion about the misunderstanding and come to an understanding. I hope your husband will look inside himself and deal with any jealousy issues and remind himself that this lifestyle is supposed to be fun. At the same time, you must expect people to be true to their word, and that's why a continuing open and honest dialog is so important.