The pros and cons of coming out of the closet...as I see it
Coming out of the closet can be a frightening, nerve-wracking thing. In my own personal circle of family and friends (not my husband's, not his girfriend's) there is truly only one person I worry about finding out. Not because I think this person will not love me, but because it may well hurt her. This person is my paternal grandmother. Granny has always been incredibly important in my life and she gets a lot of the credit for helping me to become the person I am. She is Southern Baptist, and lately her views have become a little less flexible. She is still the gentle loving person she always was, and she would never be harsh or turn away from me, but I do not want her to ever be hurt by what she might see as my sinful way of life (laugh).
I would love to be completely open about our relationship to the world. Only openess will actually move us, as a part of our society, down the road to universal acceptance. But we often balance that with how it will affect those we love.
Our choices in who knows or who doesn't often leaves us treading a thin line. The very nature of being poly means that we are perhaps more open to the feelings of others, more affected by what others think or the hurt we could cause with who we are. But we also have the RIGHT to our feelings and choices, and having to hide that brings us pain as well. For each of us, for each person we tell, we have to consider the effect that knowledge will have on them, and on others in their lives. Like any other piece of knowledge we share (and most especially life-altering, earthshaking knowledge) it is very much like droping a stone in water. The larger the stone, the bigger the wave. Some of those we tell (and those they might tell) will be swamped by the knowledge, others will ride it out and come out a bit stronger and more experienced on the other side.
Ultimately, with each telling, we have to decide how important our relationship is with that person. We must balance how much we need to be completely honest with that person with how much it will hurt us to loose that relationship or cause them pain if they cannot accept or understand.
Hmm, that was a lot of rambling and very little practical advice I am afraid. So here is the practical advice:
Try holding a philisophical discussion with her about polyamory and play 'devil's advocate' so to speak. If she is rabidly against it and will not consider that those in poly relationships might be happy and normal, for themselves, then you will know that telling her will most likely loose you that friend. If after some discussion she opens her mind, then there is a chance there.
Good luck and be strong.