Concerning the question of being and knowing that one is polyamorous: Sometimes you just don't have this great insight that you yourself show. Maybe she was never able to name it or she just didn't explore it or whatever. Don't assume that it hasn't been there before things like that happened. I am stressing that point because it has been like that for me. I 'am great with monogamy', if you want to phrase it like that, I had a satisfying relationship with my husband and loved him and he loved me and such, but I fell in love nevertheless. IF this was just a reaction of hers to the rough time she was experiencing in your relationship, the root could be something different from the basic longing for more than one relationship/love.
As far as you described it, was this about more than sex? Did she fall in love with this person or was she just searching for attention and support? Poly is all about the feelings and connection you develop with the people/partners in your life. What did she say she wants exactly? Open in the sense of being free to develop new, lasting relationships or just to have a physical outlet from time to time?
The last point that came to mind while reading about your problem would be the poly/mono issue. I have two monos with me, kind of. The main difference between them and you seem to be the need for the monogamous structure of the relationship. For them the answer to 'Would I be able/willing to live like that?' came naturally when comparing the feelings they had for me to the loss they would feel if I wouldn't be in their life. For both that answer was crystal clear and I can say that after some adjusting and getting used to things, both don't feel that much of a difference in regard to how things have been before(husband)/in other relationships (spouse).
Whatever your answers may be in the end, wishing you luck on your way and success in figuring things out for yourself.
Edit: What I forgot, the cheating part. I have been cheated on as well and for me the same system worked as for my two men. The moment I knew about what has happened (which was immediately after the deed was done) I felt deep inside that I wouldn't want to live without that person by my side. And I saw my fault in it as well. It took time to process things. But for me, things were decided within minutes after the first shock. Maybe you felt something similar and knew right away if there is a future for your relationship or not.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.
Last edited by Phy; 05-08-2012 at 07:30 AM.