am I wrong ?

So my boyfriend the other day confessed to me that 4 months before he met me him and this girl we all hangout with in our group of friends tried dating they went on 4 dates they even "hooked up" did sexual things with each other but never actually had sex ( so he tells me ) he later told her he doesn't see her as more than a friend and he isn't ready for a relationship she got pissed off at him but they're fine now she got over it. But 4 months later he met me and 3 months after that asked me to be his girlfriend. now I'm not really close to the girl my bf and I have only been dating a year now. But it bothers me now that I know this that I have to hangout with her and pretend like I don't know and be friendly with her. I expressed to him that I didn't care for the girl to begin with bc I found her to be overly ditsy on purpose and just very annoying. He seemed to have gotten upset and told me that they're all really close with each other her included. Am I being crazy for being upset about this. Bc he seems to think I am.