My biggest fear when I talk to my H is not that he'll leave to be w/OW, it's that he won't leave and still want OW. I completely understand your dilemma.
Here's the thing... He's full of sh!t. You know it. I know it. We all know it!
You can't control if he wants to be with her.
But if he wants her, he doesn't get you. Period.
Tell him that M isn't a threesome. That he has already walked out on his family by pursuing a R with somebody else. That he is setting a bad example for your kids. That he doesn't get a wife and a girlfriend.
You can validate his need for OW without endorsing it.
Good luck, I will be thinking of you!
Oh, and Merry Christmas too
Bomb January 2012 - doesn't feel the same about me
~ "There is nothing love cannot face; there is no limit to its faith, hope, and endurance."