Can cheating ever lead to healthy cuckolding?

Hi everyone.
My fiancée has admitted to sleeping with her boss for the past three months. She volunteered the information freely but was not remorseful. Her friends and sister have known for two months.
She has given me an ultimatum, either I accept it and we get married, or we break up.
I love her so much, she means everything to me and I can’t imagine life without her. At the same time, this is humiliating. I feel so ashamed.
Fundamentally, I want her to be happy, but some of her expectations are intense. For example, she’s wants him to be at the wedding.
Can this lead to a healthy relationship? Or will it build into resentment. I would appreciate your thoughts.