Yep. Been there, done that. Better and stronger than ever.
Cheating could be a sign that something is missing in the current relationship. It could also be that humans may not be wired to be monogamous.
Sometimes a relationship is broken and there's nothing to salvage with or without an affair.
I think that some people use affairs as a way to end a relationship because:
a) They won't be alone afterward
b) They can use the new person as the reason for the demise instead of saying that they just don't care for the initial person anymore
And now, for cheating-related entertainment: