A really similar situation almost ended my partner and I. And I'm actually on the other side of what a lot of people here are describing.
I was the primary partner and almost ended my relationship because of my metamour. Though I can say that I never gave an ultimatum. I was severely unhappy with my partner's choice of partner and it was seriously affecting my ability to trust him because I didn't trust HER. (and I call bullshit on the people who say "just trust your partner and things will be fine"). He was manipulative, disrespectful, and would try to make me feel like I was being a bad girlfriend because I wasn't 100% okay with her.
My partner and I didn't break up (though we seriously came close) largely because he realized on his own that he couldn't give her what she was asking. She wanted to be a "primary" partner, which was something he did not want. They're trying to work things out more casually now.
I was the primary partner and almost ended my relationship because of my metamour. Though I can say that I never gave an ultimatum. I was severely unhappy with my partner's choice of partner and it was seriously affecting my ability to trust him because I didn't trust HER. (and I call bullshit on the people who say "just trust your partner and things will be fine"). He was manipulative, disrespectful, and would try to make me feel like I was being a bad girlfriend because I wasn't 100% okay with her.
My partner and I didn't break up (though we seriously came close) largely because he realized on his own that he couldn't give her what she was asking. She wanted to be a "primary" partner, which was something he did not want. They're trying to work things out more casually now.