I had the exact thing happen to me in my last serious relationship. I told him that it bothered me that he agreed to go without talking to me about it first. I actually didn't and wouldn't have a problem with it in theory. I just think he should have said to ex, "I think I might like to but I will let you know later if I am available" and then came to me and told me that he has been asked by her and he thinks he would like to go but wanted to make sure that I had no difficulty with it. In the end, I actually did get bummed out but I got over it quickly and even helped him pick out a tie and such because I knew he really cared about me. He went with her and had a horrible time

. As I said, it would have been nice if he had of mentioned it to me first, not for permission but as a courtesy and out of respect for my feelings.