My assessment from afar is that your wife is not fully committed to the marriage and if the OM were to become available, she'd be off with him again. I think that's what's nagging you.
That's an awful thing to tell someone, and I'm often wrong, but it seems like she's doing what she needs to do until an opportunity arises (i.e. cake eating).
I hope someone comes along and tells me I'm full of sh*t.
