I haven't told my wife that "I will not live in an open marriage" probably because she hasn't admitted to anything. However, this is something I will definitely do to set a clear ground rule. She told me the other day that she has broken all contact with him which I don't believe.
Then the next time she says this, say "I wish I could believe that right now, I really do. You do need to know very clearly, while I realize I've made some mistakes in this marriage I WILL NOT live in an open one. Looks like we both have some decisions to make."
Or something similar.
She needs to know that you will call her on her ch*t, even WHILE owning up to your own mistakes and demonstrating self-improvements. If not, she will lose respect . . . which kills attraction . . . which kills love.
After you read DB or DR (I recommend DR), a couple of books I would recommend are "Boundaries," by Cloud & Townsend, and "Love and Respect" by Eggerichs.