I am in the camp of not exposing. But I do agree with others that you really painted yourself into a corner.
I was think about how each circumstance plays out:
1. You keep the secret, affair: a. fizzles out on its own b. lingers on
2. You tell the other spouse, affair: a. fizzles out on its own b. lingers on, with more deception & hiding
I see no advantage to outing the A, Ultimatums/pressure rarely seem to help anything.
Side note: document the details of the A in a very secret place. Could be require if D ever comes into play.
Me: 43 M: 10y S:15 ILYBINILWY 2/18/13 W moved out 2/18/13 Filed for D: 2/17/13 Got DB: 2/20/13 Got DR: 2/23/13 180 & LRT Began: 2/25/13 D Final Dec '13