Can I just suggest that she has thought about this in terms of her having sex with him and you also and realized how jealous/envious she would likely feel? Some people, MOST people are not willing to feel jealousy because it is too scary... it isn't something that we are used to feeling and people usually try and avoid it.
I would suggest that if this is important to you (and him!) that you keep at it... try again and tell her about poly... that we exist and it is possible. Give her some education... not yet maybe as she is sounded threatened, but in time. As I said, it all takes time. This is part of that time. There is a huge part of your life left and it is healthy to move towards change.
If keeping at talking about this means you and her are done, then that is how things go sometimes... the thing is you could be done just by virtue of the fact that now you have to sit on your loving and bonding thoughts with this man. What do you have to lose in this? Nothing! Which is a more healthy route? TALKING ABOUT IT! Its out now and you have started a path.
I realize that it is scary and you are concerned. After all, everything you know is being threatened right now. Be gentle with yourself and allow things to normalize every time this comes up... each time will move you closer to what is meant to be...
*hugs* good luck