I'm going to preface this by saying I'm still young and very much learning myself, but I'm going to give my best shot at making sense of this situation.
It sounds like maybe the jealousy hit her harder than she was ready to handle. That she said she was poly for 13 years is one thing...but time doesn't necessarily translate to experience? I'm thinking she may be asking you to consider monogamy because she is scared to lose you. By not talking you with it openly, it sounds like she is dealing with a past hurt or insecurity which she hasn't fully come to grips with yet. It's definitely understandable that you feel hurt by her not considering your feelings, but at the same time, the reason she may not be doing that is because her feelings of anguish are clouding her perceptions. When someone holds onto pain in their heart like that, it can create a wall that is hard to pull down, but must be pulled down before communication is reinstated.
My advice for you: stay calm even when your fiancee isn't. I wish there were an easy answer, but for the person who is holding onto the anger, it is entirely up to them to release it. You can't change anyone but yourself, as the saying goes. By being there and being calm and unaccusatory and unpressuring and whatnot, you help create a calm and serene environment where even if your fiancee feels threatened inside, she can be coaxed to relax and be more likely to open up. Nobody's perfect, and no relationship's perfect, and polyamory is essentially built around navigating, not avoiding, jealousy. You will both feel it, and as you are engaged, communicating is especially important, more so than just dating or being a couple. There's a promise of commitment in marriage, beyond that of normal friendships and relationships, even poly marriage. Perhaps saying that to your fiancee would help remind her where your loyalties lie and that you're not just some free bird who could fly away. I could be wrong, but it seems like part of the jealousy stems from her fear of losing you. It's a scary thing sometimes, and I think I can see well where both of your feelings on this are coming from.
I hope not too many of the things I said were projections, though I'm sure some at the end there were more personal than universal beliefs. I hope at least they resonate some. I wish you and your fiancee the best of luck in communicating and being intimate and working through this however feels right.
Last edited by Hmm; 12-14-2013 at 10:44 AM.