I can certainly say that I am very sure that group sex or swinging was NOT what I wanted. If my husband had suggested a foursome as a gateway to poly I would've said no way is that happening, and might have closed down if he didn't seem to see a different alternative. Might as well have asked me to go up on stage and masturbate for a crowd of strangers, a foursome idea is just not something that would be in my comfort zone unless I already had some close friends that I thought might be people I was open to trying it with.
I am not sure how you approached it, perhaps what she heard was "this is only OK if we have a foursome (or you masturbate on stage) first" and it was so alien to her she shut down, perhaps having some of the same feelings you had when she first brought up opening up your relationship. Maybe if you bring it up again with a preface of "I understand this might not be what you want or are interested in, but I want to talk about our feelings about all the aspects of this, and I am open to listening why you don't like my ideas, just like I hope you are open to listening to why I don't like yours, my goal isn't to talk you into anything, but to see if we can find common ground and build from there"
Have you read any any books on poly? Maybe if you get Opening Up
and agree to read it together so you have a somewhat objective voice guiding you through these conversations it might be useful. Come on, $14 bucks is cheaper than a counselor!