Here is a note on my experience, and you can take it for whatever it's worth.
I'm happiest when the person I love does not expect monogamy from me. The poly structure really suits me. (I don't really see poly as an underlying essential identity, and may be rare in that-- it's what I do, not what I am. What I am, is bad at monogamy.)
I have cheated on monogamous relationships in the past. Sometimes after 1 year, 2 years, or 7 years. Sometimes this was because the relationship really needed to end, and sometimes not.
I can't quote the studies, but if you go to Dan Savage, he can, but most people cheat. Men and women both. It's common, and you can decide whether it's worth ending the relationship because of it.
But my point is this-- in poly, I'm not going to lose a relationship because of the "ooh, new shiny!" impulse. I'm also not going to be able use cheating (mine or theirs) as an easy way to express "this relationship is broken" like I used to. I deal with a relationship strictly on the terms of it's own health, how well it's working for us, how it fulfills us, without the expectation that it be my all-and-everything, forever and ever amen. It feels more stable.
If she's talking about poly, she may be trying to change your relationship to something that she thinks will be more sturdy. It doesn't sound like that's an option with you, so make sure she knows.