Originally Posted by graviton
a. deny your spouse a second lover under threat of divorce and separation, give her the ultimatum and make her choose, this will still cause resentment for both of you. Hers because she is being denied, yours because she had the nerve to bring it into your marriage. Not to mention the paranoia this will create because you can never be sure if she just decides to have an affair instead.
b. Allow the second lover for her happiness, and attempt to come to grips with it and learn from it. This creates resentment only for you but maybe you will learn to lose that resentment.
c. Take responsibility for your own life and your feelings. Stop trying to make this a discussion about "the relationship" and bring it into terms that you actually have any real input on (your own feelings and actions). All you can do is figure out what is right for you and make decisions accordingly. If your spouse has told you that she wants to live a non-monogamous life and this is antithetical to your worldview... make a rational decision based on the facts at hand.