HELP! Avoided the Topic...?

Before my current boyfriend and I got together, which was about two years ago, I liked one of our mutual friends. The mutual friend and I would make out occasionally and I took my shirt off with him - the first time I've ever done that. The mutual friend eventually decided he wasn't looking for a relationship, so we never got together ever again. A few months later, my boyfriend and I got to know each other better and began to date. He had sex with another girl before he took my virginity, and was very honest and open with me when he told me about all of that. My boyfriend is just now finding out about what happened between me and our mutual friend, and I'm not sure if he's mad or not. Do I have to have a reason why I didn't tell him? I never lied about it, I just never mentioned it. Am I in the wrong? What am I supposed to say to him?