It sounds to me, that minus the ultimatum, it's the same situation you went through with your bf all over again.
I mean she knew you were poly from the beginning right? I understand that sometimes logical knowledge is different than being confronted with it emotionally. Still, she fell in love with you - who you are - and yet she is not permitting you to be who you are. It's like loving a musician and never allowing him to play for anyone but you, or loving a painter and permitting her show her work only to you. Hoarding something or someone does not make it more precious.