If a marriage got better afterwards, I'd say it was in spite of the adultery,
and that is a BIG IF....
People are too scared to leave, they would rather be with a cheat than alone...STUPID.
They call themselves in LOVE, so they FORGIVE.....(or so they say)
There is never a one time cheat....Just like a one time theft....
They just have to say something to make themselves feel better for staying and doing something they KNOW is stupid.
Even the Bible lets you off the hook if they cheat on you..I know of no plainer message of GET OUT NOW (when they cheat) than that.