Pru, I love that you said "No" to your husband's request for more time. It doesn't strike me as impulsive at all. I am not usually one for lots of rules in relationships, but it is important to honor agreements. And agreements shouldn't get revised without forethought and discussion. Time management is an important skill in polyamory. To have said, "Sure, have more time" would be like giving in to him just because he messed up and now wants what he wants, like a little kid. And you probably would have been seething afterwards, which is not good at all, because you would have gone against what you wanted and then felt manipulated or victimized by him, and betrayed by yourself. Doing something like that would not have set up a good, workable precedent for the future.
Agreements are agreements, and breaking agreements results in consequences. I am sure he will appreciate that fact eventually, just as I am sure he appreciates your having encouraged him to take more time to be with her another night. You did well in establishing a strong foundation for poly, I feel. I would consider this whole "incident" and how you handled it a great, big poly success!