I still don't like your subject line, but you at least admit in your response that infidelity is a choice a person makes, not a simple cause/effect. I don't like to see infidelity diminished and dismissed as the fault of the one betrayed, particularly since in many cases, if not most (yours obviously not included) the tempted or cheating spouse did not effectively communicate the problem prior to choosing the low road. I also disagree completely that a life with little to no sex is a meaningless life. That seems overly dramatic a statement to me. I suppose that if sex is the only part of your life that is meaningful, then by definition your life would be worthless after the sex is gone. But if sex is your only reason for living, then I think you need to create a more meaningful life for yourself.
LL: I agree that the LD/ND spouse probably shouldn't be surprised by the presence of an A. Nonetheless, it almost always comes as a shock. Only hindsight is 20/20 for most of us.