If you have come across to your fiance the same way you have here I can see why she would feel threatened. When I was reading your original post it was sounding very much as though you view women as playthings. I imagine in her mind she's wondering if she is one of those playthings.
I would suggest for a while backing off from the whole idea of poly and focus on your fiance and tell her what's special about her, outside of the way she looks or how fabulous she is in bed. Then as you begin to reintroduce the idea of poly explain to her what you see in the other women that doesn't have anything to do with the way they look or how they might be able to satisfy you in bed. At this point it's probably going to take a long time to rebuild that trust with your fiancee. Set a time with her, maybe around the time of your child's first birthday, to reopen the poly discussion.
Everything will be ok in the end. If it's not ok it's not the end.