I come at this from the perspective that infidelity, deceiving one's partner and denying them choices in life are fundamentally wrong behaviour choices. I also think good people do things that are wrong. I think it's a natural part of our self-preservation in life to justify to ourselves why we are doing something that a) we believe is wrong and b) we wouldn't like done to us. So we make excuses, many of which appear on this thread. Others come on to collude with those excuses and justifications - and I don't think it is necessarily helpful to those of you in such turmoil, which I sympathise with, from a humanitarian point of view.
Those of you (OP specifically) who were honest enough to admit that your marriages were happy before you embarked upon an affair are further down the route to understanding why your affairs happened.
Although this is painful, I think you might gain better understanding of your predicament by becoming much more introspective. What is it about you that led you to this behaviour? Could there be any narcissistic traits in you that have been previously unacknowledged? Are you punitive (he won't have sex/propose marriage/talk about feelings)? What have you been giving to your primary relationship (as opposed to "getting")? If your primary relationship was unhappy and you believe this to be the reason you had an affair, why did you not make a different behaviour choice - to bring your grievances to the table, compromise or indeed, leave the relationship?
I see many times on here people justifying not leaving primary relationships "because of the children". I think that's worth challenging too - how much is it really about the children, and how much is it about other things that you value, such as status, society's approval and in some cases, financial survival?
I can perhaps feel less emotionally charged by people's need to find excuses for infidelity because fortunately, when my H was unfaithful he didn't try to re-write history and claim that there were problems in our marriage - and he never once blamed me for his affair. Healthily for him, me and our marriage, he recognised pretty quickly that this was a fault in him and that infidelity and deceit are never justifiable. So he went about fixing himself and in turn, we were able to rebuild our marriage.
You will however see other men and women on infidelity threads who resent deeply that their unfaithful spouses re-write history, apportion blame in every direction but themselves - and this compounds the hurt.
I also think for those of you who are conflicted about whether the OM is the real deal should try to do a bit of projecting into the future. What if OM one day withdraws sex, becomes selfish - or indeed displays any of the behaviours that you believe are present in your current spouses? Will you have another affair with someone else, or will you learn from this and resolve to express your dissatisfaction in different ways?
The language used in many of the posts is revealing - words and phrases like "victim", "swept away", "can't believe this has happened to me", "powerless" - all of these put you in passive mode and evade your own responsibility for your own behaviour.
Good people do bad things, but if we do, it's about us - it's not someone else's fault and it's a choice we make. The really difficult stuff is about being introspective enough to own the bad behaviour and get to the root of why we're doing it - to ourselves and the people in our lives.