HELP...Am i wrong or is he over reacting?

I have been dating my current boyfriend for nine months. I am fully committed to our relationship and I trust him completely. A couple nights ago I stupidly decided to tell him that when we were first dating I kissed another guy. This happened really early in the relationship when things were still very casual and we hadn't discussed being exclusive. At this time we had only been on 6 or 7 dates. He is having a really hard time dealing with the news and wants to talk about it all the time. He basically told me that his image of me has been somewhat tarnished (I was really upset by this). I really didn't think it was that big of a deal, but apparently I was wrong. I haven't seen this other guy since the night I kissed him, and I have no intention of being with anyone but my boyfriend. Is he over reacting? I am wrong to tell him that I don't want to talk about it anymore?