Amelia - you are right when you say that people do have affairs that are never discovered and that sometimes this is the best solution. I am one of those people, having had two undiscovered affairs a few years ago. I had those affairs not because I was unhappily married, but for the reasons that Charbon described upthread; an ego-boost, the chance to have sex with another man and for the adrenaline that the illictness provides.
But I didn?t stick my head in the sand when my affairs ended (I ended one, OM ended the other one.) I took responsibility for what I had done and sought counselling to understand why I had chosen to be unfaithful so that I could stop it happening again. And yes I do live with the guilt of my undiscovered affairs. But I chose to have those affairs, and I now have to live with the consequences of my choice.
You also said, Amelia that you feel that other posters had doubted the OP?s DH?s reaction when he blamed the OM. Surely the important point here is that the OP shouldn?t allow her DH to let her off the hook, even if he is trying to protect himself. As Pfft said he is taking the line of least resistance and it?s upto the OP to put him right about the choice she made so that there are no further lies between her and her DH.
Disambiguation - you said that you assumed that MNetters would like to see the marriage survive and that her DH should believe what he wants to, in order for this to happen. I want to see this marriage survive, but the only way this can happen, now that OPs affair has been discovered, is if she is totally honest with him about the choices she made, rather than going along with his head in the sand approach that she was ?taken advantage of?. She also has a responsibility to herself to get to the bottom of why she cheated. If both parties live in a state of denial then there is the danger that this will happen again.
It may be that the marriage doesn?t survive. It may be that there are tensions in the marriage that have contributed to the OPs decision to look for sex outside of her marriage. What is important is that any further discussions are conducted in openness and honesty, with no secrets or half truths so that they can both decide whether or not to continue their relationship.
I don?t think that the majority of posters here are being self-righteous. Many of the responses here are borne out of the pain of adultery experienced by DW/DH and OW/OM