Knowing my wife, aggressive behavior or having ultimatums will backfire in a heart beat, and I believe more passive behavior will work best.
WRONG. This will not work.
Reasonable people can differ about different aspects of DBing and affair-busting, but I'm pretty certain that "passive" isn't in the description of ANY of the successful methods, or their various hybrids.
There is a whoooooooole range of behaviors, stances, statements and actions that you can make in between the black of "moping"/"on the verge of meltown"/controlling/accusatory, and the whiteof "calm, confident, assertive and detached."
It is in that GREYthat you will find your most successful approach, Dan.