I haven't seen anyone here who was "left in the dust" because of an OW or OM. The OW and OM made it into the M BECAUSE of the problems in the M, they didn't cause them.
A vandal might only get into your home because you've left the door unlocked, but the damage he causes is on his head alone. Your negligence doesn't absolve him from his destruction of what is precious to you.
I'm sorry Oldtimer, but I have zero sympathy for OM/OW. They are -- by definition -- PREDATORS. Yes, they usually find their way into dysfunctional marriages, but:
a) Such a marriage deserves an outsider's particular tenderness and respect, not their intervention and destruction; and
b) According to research by Dr. Harley (and others), this is simply untrue in 20-25% of the cases. Harley found that EVEN HAPPY, HEALTHY MARRIAGES can fall prey to the lure of infidelity -- a finding which he said shocked him. It is a myth that all or even "nearly all" marriages which include affairs are dysfunctional, and even dysfunction is no excuse for a unhappy spouse to not say "We need to talk," before they spread their legs for another.