In my sitch, I was given some advice about such "talks" and it served me well:
Tell her "There's really only two things to talk about: our marital relationship, and the END of our marital relationship. I'm not going to discuss the former so long as you've invited a 3rd person into our marriage (alt. version: "so long as you're having an affair"), and as for the second, that's really best handled by attorneys, I think." Then you add, without pausing, "Now if you'll excuse me, I'm going for a run," or "I'm going to go get a workout in," or "I have another appointment I need to get to," and you excuse yourself.
