Popzie - I have nothing much to add to what I said earlier, but I hope my posts have shown I am not judging you, or that I have no sympathy for your situation. This post is however for VCAU and Spaceman.
Spaceman, of course my advice would have been different (in part)if the true story had been originally posted. The advice about bringing the lack of intimacy to the table (in a different way) still stands however. Understandably, if Popzie had mentioned the OM in her original post, I wouldn't have advised her to say "I'll never be unfaithful to you.." as that would have been absurd. And I agree with the light analogy downthread - the OM clouds the picture enormously - it is virtually impossible to judge our partner's qualities objectively when we have developed an emotional connection with someone else. It follows therefore that the advice we give will always be different if another lover is in the mix - this is not judging, but it is pertinent to the story.
VCAU - I have no idea where you get your evidence that "most relationship advisers counsel against" being honest about past (or current)indiscretions. I wouldn't dream of telling posters that my belief - that honesty is best, was based on anything other than my experience with my own 25-year relationship, the experiences of my friends - and my husband's eminent psychotherapist. I could post links all day long here from counsellors and relationship authors that echo my view, but I prefer to talk about personal experience. Which is that keeping secrets damages intimacy in relationships.
As for telling all being "self-indulgent" - now this I find truly absurd. What could be more self-indulgent than deluding someone that you have always been faithful and always intend to be? Or not having the courage to lose the financial contribution of your spouse and the status of being married and instead taking those things from a trusting partner while you continue to pursue "other arrangements"?
I understand all too well why people have overlaps in relationships - and why they fear telling their spouses that they have been unfaithful. But kidding oneself that the only reason for keeping the secret is so as not to unnecessarily hurt the unknowing spouse is one of the biggest delusions of all. The main reason people don't tell their spouse is because they fear the rug will be pulled out from under them - often erroneously I think - but I understand why people believe that.
And the reasons that so many relationships overlap is because people will often "make do" until someone better comes along, either for emotional reasons or financial; sometimes both. All of this is entirely understandable - and while this is not an honest, transparent way of behaving, I can very much understand why this happens.
To pretend that keeping secrets is in the name of some noble cause "not to hurt" is frankly delusional IMO. While that might be a part of the reason, it is certainly not the whole story.
Popzie - not much of this relates to you, I'm afraid. It sounds as though you are willing to financially support yourself and are willing to live on your own - and no-one should be forced to live with a mistake they made in their twenties. In your case, I suspect this is a classic "exit affair" and if that's the case, by the sounds of it you would prefer to live honestly without deceit and lies. I hope that's the case - deceiving someone is always wrong I think, because it denies them their choices in life.