We’d all like a smoking gun. We don’t have one yet. So what are the most constructive things we can do? Confront? This will build anger and resentment and entrench them with each other deeper, so no, not a good idea. You know this you said this here.
I disagree. Confronting and exposing affairs is designed precisely to drive the affair partners together, and to take the romanticism and mystique off of their relationship. By temporarily driving them together, you are allowing them to see each other's warts, faults and stresses, while YOU are busy becoming the better option, whether it's in THIS relationship or your future one(s).
People say "it will only drive them closer together." Um, that's the general idea.
Starsky [edited by dbmod to add note: exposing the affair can be dangerous to your marriage and is NOT recommended DB advice]