I couldn't just drop this but know I would have to move on from it. I have access to his phone if I want it although having read lots of MNET threads I know if people want to cheat they will find a way, he'll use another phone, email account whatever. I'm pleased he had no issue with being open but I don't feel that having that openness means he won't cheat. I need to feel that the hottest woman alive could walk up to him and say "take me and no-one will ever know" and that he would walk away. I also don't want to be a phone checking paranoid freak. So I see why it's important but don't think it's the answer.
Yes aka we did discuss fidelity and faithfulness and we both agreed that "cheating" was anything that you wouldn't tell your partner about... so texting, inappropriate conversations etc. We both agreed that that was where our 'line' was.
At the time he gave me his explanation of "low self esteem" due to everything that was going on post him leaving and still says that is the case but has opened up more about why.... deep breath for me...
He has always been intrigued and interested and turned on by the fantasy of promiscuity/group sex. (I knew this anyway as he has told me and he said for the last year of his marriage before he met me and there was no sex at home he had thought about swinging etc but never acted on it.) I always just thought it was a fantasy. I have fantasies which I would NEVER take further! He says in reality what he wants is to be with one person and love them in a relationship.
He said he has never let it become a problem or crossed any line. Never cheated in his marriage till he met me. Says cheating with me was about falling in love and wanting to be with me.
He says when he goes on a night out boozing with his mates he has to manage himself by doing certain things... he always reminds himself how he wants to behave beforehand and if he ever chats to a woman he talks about me upfront and then likes being in contact with me on the night out (text etc.) He says all these things stop him from crossing a line.
He says on that night he didn't remind himself of how he wanted to behave before he went out and was talking to this woman about me and how great things were and ended up talking about our sex life (which he now tells me he agrees was inappropriate) and apparently she "liked the sound of me sexually" 
. I asked if he was trying to arrange a threesome (I have told him in the past this is one of my fantasies although NEVER gone further than talking about it before/during sex.) He says no but they did talk about it and it mentally turned him on. He says he was given her number by his mates (yes this is a bunch of 35-45 year old men... tragic) and the reason he didn't tell them he was serious with me is that they know his ex and he was worried it would look like he'd had an affair (he feels deep guilt and shame for this.)
Anyway - he doesn't know why he then texted when he got home, says he was drunk and stupid and his head was still in that silly place of fantasies.He said he would have never gone on to do anything as the next day he was ashamed of himself and what he had done.
So it seems two different forms of cheating and two different reasons for it:
- According to him an affair with me was for different reasons to this. He says he just fell in love, and is in love and feels like we should have met in the first place and wishes we had. (Our affair was emotional only for AGES and he's a good looking guy so if he'd have been wanting to get his end away he could have done it in many other ways.) In addition nobody ever found out and we did talk a lot about whether/how we could both make our own marriages work. He left his W voluntarily and not cause she found out or I asked him to so I do believe that the affair was about me, not him looking for a shag or some fun.
- This "fantasy" (that crossed the line.)
He says the drivers of him doing what he did were drinking, egging on, poor self esteem (feeling awful for the hurt he cause ex and kids and me by hiding me away) and the wrong state of mind.
He says he's always had fantasies of group sex and sexual promiscuity but has to manage and occasionally suppress them and has been doing it for years and is "good" at it.
Says he is sorry and can't undo it but he knows he can manage himself and is capable of a real and honest relationship. Says all the secrecy and hiding me was a function of our affair and the bad place mentally that he got into as a result of it.
I am pleased he has finally been honest (although not sure pleased is the right word.) But now can't shake the fact that when he goes out drinking with his mates (probably a 1/4ly thing) he has to do certain things to "manage" himself. Is it ok as he knows that and does them or is it (as I now think) a bit weird that knowing you're in a relationship isn't enough to stop you crossing a line.... or that being in a relationship should mean you don't WANT to cross a line (although since reading lots of mumsnet threads I am more cynical about that with any man.)
We have no plans to see eachother. Indeed I asked him on the phone "so would you tell your next girlfriend about you having an affair and this texting thing" and he just said "I don't want another girlfriend. If you've decided that then I better go". I told him I wanted to know the answer and he repeated "I don't want another girlfriend and I really think I need to sort all this out if I am ever going to."