Based on what's been said, I could see how it would be embarassing for him. If the perception is that she jumped out of bed to go talk to another guy, and then was overheard loudly flirting with him, I could understand her boyfriend being upset and feeling disrespected in front of other people.
For example, my boyfriend is very touchy. He doesn't mean anything by it, but he acts that way with people he likes and feels comfortable with. There have been times when I've been embarassed because other people around us see his actions, presume him to be flirting or making a pass right in front of me, and then start giving me looks. He and I had a talk about it, and I expressed my worry that he might be giving both onlookers and his female friends the wrong impression by being overly touchy. Even though he clearly wasn't flirting, he agreed that he needed to be more cautious about the message he was sending out.
~Break the Chains of Your Mind and Your Body will Follow~
I can't argue with you on that...I am naturally and OUTGOING person, and to some, that can be taken as being friendly - to others - it may come across as being flirtatious. Would I EVER, blatantly and maliciously FLIRT with other guys with/without BF there? NO. That IS childish behavior, and flat out disrespectful on many levels.
These friends of mine were in the picture WAY before BF was, and they were fine, like you said with your buddies...you could fool around in 'jest' but nothing came of it, and nothing would either. BUT, unfortunately these guys have been nice and pleasant all up till I told them I was seeing someone - THAT is when all the tables turned, because they went from friendly to 'weird'. Would I still try and hang out with those same guys on a regular basis as I had before BF, again, NO. But I saw nothing wrong with still associating here and there with those people - as long as THEY understood that I wasn't interested and was dating. I was talking to my cousin (male) about this situation - and he said the same thing you did....he doesn't condone BF's 'grudge' going on for days, BUT, he does see how MY behavior could lead to any guy being insecure or having an issue with my involvement with so many guys - as closely as I had been! And for me to ignore that WOULD be immature...I understand that. Which was the purpose for this OP - cause I know I was wrong for what my part, and I realize that my past actions/behaviors could 'insight' BF's insecurity.
You seem to be a very fair person. The only thing you can do or are obligated to do right now is to have one final, extensive talk with him about it. Stop apologising after that. If he keeps the grudge up, I'd tell him where he can step off. Nobody needs that in their life.