Interesting thread for AIBU... Have namechanged -oviously
I was the classic "foolish" OW (I'll take foolish over stupid). I apologise if this is a confused post - I have very much compartmentalised what happened and do not think about it too much. My thoughts are therefore a bit chaotic. ANd I've had a pinot or two!
Relationship affair ended over 5 years ago. It still impacts on my life. Quite aside from the practical - I moved house, jobs etc to get away from it all - the emotional aspect. It was the closest I ever came to suicidal. Too close for comfort.
I knew he was married. I knew him for about 4-5 years before we began an affair and over that time we became very good friends - I'd say he was one of my 2/3 best friends and so I trusted him. So I believed the shit that he told me. Of course as with all good lies it helps that they are at least 50% truth. And that he believed much of it himself. The affair happened at a real crisis point in his life - marriage going badly, career going badly etc etc I was probably flattered at being seen as being the one good thing in his life.
With regard to the hiding and getting only 50% of him. There were times that was true and times it was not. We worked together and so that gave us a huge amount in common (especially compared to his non-working wife) and why workplace affairs are such a danger. Also. we were good friends, went to lots of mutual events, his wife was very unsociable, tended not to show up so it was kind of natural that we would spend a lot of time together. I'm not one for PDA's anyway so it was just like many many workplace romances for the most part. Then there were the times that killed me - like when he and his wife went away for their 10th wedding anniversary... I never had to deal with a Christmas but I do remember not telling him when it was my birthday because I did not want to make him feel...obligated.
I knew, very vaguely, his wife. I did not like her (seemed to be mutual) and thought for some time that they were very unsuited and that he was unhappy. And to a large extent he was and their marriage is and was unsatisfactory and not one that a lot of people would settle for. On the other hand it is unhappy and unsatisfactory and very much one that a lot of people do settle for. I suppose he was dreaming of being the kind of person who did not settle but in the end he did - for a relationship that he did not really care about, a job that he hated and a life that he did not want. Was he a coward? 
Was he honourable? 
He did leave his wife and went to stay with his parents. But she was so dependant upon him. They had no children but she had given up work to start her own business but had not done so for whatever reason so did not work any more, had no family living nearby, no friends. She threatened suicide, his parents expressed their disappointment in him. He went back after a week. I cut contact so far as possible and he respected that - for a short while. Cue me moving house and jobs to get away.
I got into an affair when I believed it had a future. I got out when I realised it did not. I despise him. I despise myself. I also (prepare to be flamed) dspise his wife.
People do all sorts and believe all sorts of stupid/foolish things when they are falling in love. The Relationships board is littered with such stories. It is not any different if you are an OW. Maybe it should be. I really believed I was the kind of person who would NEVER get involved with a married man. Because of that of course I fell in to it all the harder. As WWIFN says on this board (albeit in respect of the cheating husbands), because you believe you never would you have to justify yourself all the harder - so you are soul mates, star crossed lovers (I think I would have comitted suicide to Elton John singing "Written in the Stars" - sorry gallows humour and yes, IABU for that 
) and no-one else has ever felt like this.
I think one of the hardest points (in getting over it) was when one of my friends began a relationship with a married man. I was so angry - she was so cavalier and after she had seen what I went through! And then I had to listen to her saying aloud all the stupid shit that I had felt and believed to be so unique 
You make an interesting point about RL. Most people I know (beyond acquaintances) in RL know about me being an OW. I had to tell my friends becuase it nearly destroyed me. I met nothing but sympathy from my RL friends. If your only knowledge of the world was mumsnet you would think that people routinely disowned their friends if ever they found out that they were having an affair. People in RL seem to see shades of grey much better. I don't talk about this much these days but did recently confess my OW past to a new bunch of friends. Some were clearly shocked. Most were openly supportive but some quiet and presumably disapprovind but still - none have disowned me or treated me any differently after the end of the conversation.
More wine...