Given how long my marriage has been sexless, I would actually find a poignantly refreshing side to discovering that my wife had had a torrid sexual affair with another man. At least then I'd know I'm dealing with a very different problem and I would be able to ask her a whole lot of different questions. In fact, I even joke with her about it sometimes (that if she's not doing it with me, she must be doing it with someone else), but she just rolls her eyes. As if I can't stop thinking about sex, no matter what the angle is.
And, yes, in our case, sex is really the only big issue, and we are both very hesitant to blow up everything just because that one thing doesn't work.
We all know that a divorce or an affair will work. Doesn't even need mentioning. Like I can guarantee you I can immediately repair your car by buying a new one.