What the hell is my boyfriends problem?

We have been together for a quite a while. There is this girl that he has told me is one of his best friends but she lives in Europe. One week she came into town and he wanted to go and see her so we went to where she was staying and had dinner, hugs, kisses on the cheek hello, etc. I knew that back when they were say 15 yrs old they had a relationship. However I have more recently found out that things had gone on for years between them. He has broken up with other girls for her she has cheated on bfs with him. He flew to europe because he wanted to be with her but it didnt work out. I also just now found out that they slept together two days before me and him first did and were already interested and talking to each other at that point. So it is a lot more substantial than i was ever told. I am enraged that I went to see her and the whole nine yards without being told the full fledged situation between them. If that were the case I would have said hell no I am not going to go hang out with her and by the way cut the "she is my really good friend" BS out. I feel like I wasn't actually given a real choice in that situation bc I wasn't told the whole story in the first place. What do you think...am I mad for a good reason or not? Would you want to be in that situation with your bf/gfs ex with the whole story attached to it?