I am struggling with my husband somewhat on this note, too. I sprung the surprise on him May of last year.
I will voice something my oldest brother mentioned to me....marriage should be treated as a partnership. As partners, you both are supposed to come to major changes in your life together. It should not be dictated by her or by you. If it did not start out this way, hopefully you two have taken steps since then in embarking on this journey together. If monogamy is not an option for her, I can see how it does make you feel coerced. You probably feel like your choice was either go along with it or face divorce. It seems you could benefit from discussing your feelings with her further, and emphasize the need for you both to discuss needs and wants. You are partners, after all. Then maybe negotiate from there how you both can find a happier place. The compromises will be tough but hopefully they should serve both you and her, as well as the other person she cares for. I wish you the best of luck.
Me: K, female, 27. Married to Y for over 4 yrs (male, monogamous, 33). Opened relationship to E (male, monogamous, 27) in a relationship vee.