One thing that might help is something my husband has come to accept. I am the poly person in our relationship although we have yet to fully achieve a third for our "V".
Love is not a zero-sum game. Meaning, just because someone else is receiving love from your partner, doesn't mean you are loved any less. This is where the silimarity to having more than one child come into play, would you love child A any less when child B is born or comes into your life? Did you love your partner any less when you found yourself falling for the other woman you mentioned?
And for most people COMPERSION (that c word
) takes time and effort, especially if you are not used to a poly lifestyle. My husband is working on it for me and I know that if he ever choose to find another partner, I would have to work hard to have it for him even though I am of the poly persuasion.
Whatever happens, talk about it, be honest and open. If you are uncomfortable tell her and most importantly figure out why. Then you can ask yourself if it is a good reason or a fear that can be attacked and destroyed. If you do choose to go and attempt a poly relationship, go SLOW.
Things to think about, are you threatened by any male friends (plutonic) that she has? Why or why not? I am also curious as to what it is about men that you find threatening.
Good luck and keep thinking and questioning!