You should definitely find out and expose if it hasn't been already. All you're doing is establishing a boundary.
You are still married and he needs to remember that. You are not responsible for his happiness. Do it also as an example for your kids. You're protecting them as well.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.