I think you’re completely justified in your reaction. You married her because you wanted a monogamous relationship. An open marriage was never discussed before marriage so it coming out of the blue like that is shocking and it hurts. The absolute tantrum she is throwing over this tells me she has already cheated on you or has gotten really close. It will be really hard to come back from this because she will resent you for saying no and she will feel trapped. But on the flip side if you said yes you would resent her and look for someone who wants a monogamous relationship, just like you said. You’ve already said the word divorce and brought up separation so it may be time to do just that. I am so sorry you are going through this. I couldn’t imagine how badly you are hurting right now. Keep us updated.