hamster-bamster, thank you very much for your response.

His first language is English, Canadian born. I am Chinese. And "do not kiss her in front of me" were the exact words.

I won't mind his flirtlyness , if he's up front about having a girlfriend, and not put himself into situation to cause complication, but it seems like he failed to do both. He seemed apologetic and kind of acted like he got kicked in the butt and realized he was stupid, so I am not sure if he will actually learn the lesson after all this?
Cause at times I feel like he is trying because I do see effort being put into his behavior.

I became exclusive with him again because I saw him changing a lot of his behavior. But when the incident about his coworker/friend occurred, all the past memories came back.