Is this being a psycho girlfriend? :(?

So my boyfriend and I of 6 months (I've known him for two years though) have been having some real problems. A week ago he and his girl 'best friend' were hanging out in a group of four people. They've been friends for six years. The other two people in that group were dating. He and I have been well aware of the fact that she is interested in him before this hangout thing. Well, they were at the girl's house and watching a movie, when all of a sudden, the girl best friend kissed him without him expecting it. When he got home, he immediately called me and told me what happened and that he told her that it wasn't right, which I appreciate because it was good and honest. However, after this, I asked him not to associate with her anymore. I didn't want him to be friends with her because I know that she will do something again; she has a history of trying to flirt with him. I asked him not to be friends with her, but he would not comply. He has this unnatural care for her, a kind of care that should be reserved for someone in a relationship. I told him that if he were still friends with her, then I could not continue this relationship because it proves to me that he has other priorities that are higher than the priority that is our relationship. Today I asked him if I could contact her. He responded with a "I wouldn't do that if I were you". It sounded as though he was almost threatening me. So, being the girl that I am, I contacted her anyway because it's what I felt I should do. My message to her was not malicious at all. It was a message that was truth-seeking, because that's what I want, the truth. I just want to know why he can't choose between her and me? Is this bad?