I think gf always came at this relationship from a point of insecurity. I think this relationship does tend to call up feelings of insecurity. I know it does for me.
She wanted to be polyamorous. She wanted to date a couple. (Unicorn? ;-) ) Monogamy did not work for her. But she wanted the instant stability and simplicity and security of monogamy too. She instantly wanted all of our time and energy to be involved the three of us together without any of being more "important" than the others. She wanted to know, for sure, that my fiance and I would not choose each other over her.
A huge problem early in our relationship was that my fiance undermined my belief that I could and should talk to her about how much she was staying at our house. He said that we shouldn't upset her and he would find some way to say it (read: that would be better than my way) that wouldn't upset her. But he never did. He let it go on and on. I was going nuts. Now he does not want to upset her about the wedding and wants us to avoid talking about it in front of her since it upsets her.
My fiance and I are not ones to emphasize her current sort of secondary status, but neither does she have primary status just by the nature of things, without any of us deciding it would be so. Not that she couldn't ooch in the primary direction over time.
I need to put my foot down about the wedding. I need to be free to work on it and I need his help. I probably need to talk to her directly about it and explain that doing it this way is a real problem for us and the upcoming wedding. I guess I've been allowing my fiance to again dictate the communication with her. Oops.
I don't think, having so far done very little of the planning, that my fiance realizes how much work it takes to do this thing. He looked at the mile long list of things yet to do, a gazillion miles long, and said "no problem." And I'm still the one that has to be the bad guy and enforce no-GF-we-have-to-work-on-the-wedding-nights. Argh.