Here's the flaw in the "All I can do is hope they do the right thing" approach, as I see it:
1. You are dealing with someone who is in an active affair. Physiologically, this does things to their brain -- that's a medical fact. It even shows up on CAT scans, and it's what made an intelligent and otherwise sane female astronaut drive from Texas to Florida WEARING AN ADULT DIAPER so she wouldn't have to take bathroom stops, so she could get there faster to avenge her man.
2. People who are chemically addicted to their affair do NOT have their marriage's best interests at heart.
3. If you don't insist that they end the affair first, or at least lay out a personal boundary of "I will not live in an open marriage," backed up with consequences, then you are asking someone who is chemically addicted, who does not have your marriage's best interest's at heart, TO DO THE RIGHT THING.
Simply ain't gonna happen.
It MIGHT happen if the OM/OW dumps them, in which case they may use the dumping as an excuse to come crawling back, but you're still going to be one contrite phone call from OM/OW away from losing them again.
When you take the "Little Bo-Peep" Approach (you know, "leave them alone, and they'll come home, wagging their tails behind them), you are expecting an alcoholic to "do the right thing" and voluntarily pour their liquor out. You're asking a pyromaniac to "come to their senses" and get rid of their matches. Ain't gonna happen.
DISCLAIMER:
I state the above with the obvious exception of a full religious epiphany, and while I believe that God is still in the Miracle business, I also believe He gave us the tools (His Word, His Holy Spirit, friends/family, etc.) to work for things on our own, while asking His help.
