We had an open relationship for over 2 years. One where we agreed complete honesty and communication was the only thing we asked of each other. Over time, the sexual intimacy cooled down and we haven't had sex the last few months. Initially I was sick, then stressed about work, and then he cooled of because he was going through a lot of stress (which he was t the time).
Through all this, I knew that he had one long-term girlfriend, who was married and who he knew long before we ever met. Over the last 6-8 months, I have been increasingly having this feeling this feeling that something was not right and that he wasn't telling me something. I asked him a couple of times, and he told me I was imagining things, which made me start questioning my instincts.
Yesterday I accidentally saw a letter a woman had written to him, a letter that makes it very clear he had been having an emotional and sexual relationship with her. I asked him about it calmly and he initially denied it, but given undisputable evidence, he opened up and said he has been seeing 3 other women on and off over the last year.
I am stumped. I have always been happy for him to find new people, and I do not have issues with jealousy. I would have been completely okay with him dating these women. The only thing I asked for upfront in our relationship was honesty, because I have challenges with trusting people.
I have spent the last day gobsmacked. Why would he have kept this from me the last 6 months? I am left wondering if this was a subconscious way to pay me back for something he is angry with me about? I'm really not sure what to do or say. I'm not upset, I'm just really, really confused. 
[This message edited by Eartham at 5:17 PM, December 26th (Thursday)]