I am 31, he is 26. He is very impulsive, optimistic, not a fine details kind of chap.
He says he didn't want to tell me because he doesn't like to upset me, I have made it very clear that the way to not upset me is not to *do* the upsetting thing, not talking about it only makes it worse.
He didn't tell me about having sex with BF because... he told me he had kissed her, and stopped there, I was surprised, as I thought theirs was a friends with beatings and perhaps benefits arrangement, a lone kiss to me speaks romance. If he had told me they kissed, and had kinky sex, I wouldn't have been surprised (does that make sense? I've had lots of friends with benefits and never felt inclined to snog them outside of the sex window) and so he thought I would react badly if he said about the sex. I have no idea why he went on to have sex with her twice more if he thought I wasn't OK with them having sex anymore. He is a lot like a naughty boy about it actually, he was really anxious when we talked last night and his body language, tics etc, seemed very childish to me.
Ice, I think that's right on the mark. Yesterday I actually wrote out how I was feeling and listed the parameters I felt we needed to work within to put things right, including going exclusive for a while, full disclosure and having a cooling off period after discussions about changes to our relationship where nothing would happen, so we could digest, consider and speak to third parties. The "big" conversation where the stuff about previous cheating came out was supposed to be a clean slate job, as in, lets get rid of all the lies and start afresh. I wasn't expecting to hear that he had been hiding sexual liasons for 2 years.