Not sure of what is going on

My partner and I are very open with each and can discuss anything. We have been together for over 7 years. We were both married before (to other people obviously) and we both left our spouses to be with each other. It was not an easy ride, there was loads of heartache, fighting, family trouble etc... I won't go into all that detail, but all in all we are,today, were we wanted to be... happy and in love. Over the years we spoke about all sorts of staff and the 3some topic came up very often, mainly as a fantasy and not as something we were looking forward to do or not to do. We both discussed our views and I always spoke genuinely. I told him I had no idea how I would handle the situation as I had never done it before... he always seemed to be more knowledgeable, which made me ask him if he had ever engaged in a 3some. He always denied... up until a few weeks ago, when the subject came back on and he admitted having had a 3some and he gave me all the details. At first I felt good that he had opened up to me and shared his secret, but thinking about it, I now feel as though he has betrayed me. I don't know why I feel like this. Really and truly there is nothing wrong with what he has done, but the fact that he lied to me when I asked him about it makes me think that there might be more he has kept hidden from me. I also showed him my concern that there might be more hidden and that I wasn't feeling very secure anymore, but, he told me that What ever there is, it is part of his past and I shouldn't give it too much thought. Am I over analyzing things ? I really appreciate your comments... this is not something I can discuss freely with anyone!

SUSIEDQ, Thanks, You are very right.