I understand your frustration, I think it is for every couple to set their own boundaries. I have had great and also terrible times in my relationships, and I will say if there is anything I have learned it is to just say what I feel. I mean, sure at times it seems pretty reasonable to say something is just clean fun, it isn't sex after all, but if it makes me feel bad or insecure, I shouldn't keep that to myself. I would think that my definitions of relationships would make other people call me possessive, and to be honest, I think I am fine with that, after all I follow the 'rules' too.
I hope everything works out good for you, chances are that after 6+ years she won't be throwing in the towel, over some random guy, but it could possibly be indicative of a problem somewhere in the relationship. Problems can almost always be fixed, given that both parties have motivation though, and that is a good thing.
One area that does concern me, is that you say at one point she wanted an open relationship, but you said that you are rather attached to monogamy; this seems a bit of a mismatch of ideals, did you ever talk things out and come to a clear agreement on where things will stand for you to be happy with things?