1) Are you wanting to share your poly thoughts, crushes, readings, movies, and inner life STUFF with her? And would that satisfy you -- to have some poly expression and be loved as the whole you within a closed polyship of 2?
2) Are you wanting to start dating Another? And seek a polyship that is larger than the 2 people you are right now?
Because to me that's the crux there.
1) she might be able to Open herself up to. In a closed polyship of 2 -- be Open in Mind and Heart so she can share in your inner life with you.
2) That is asking her to Open in Mind, Heart, AND Body and Soul and be happy with sharing you with Another. And that she might not ever dig.
So you guys need to clarify what's on the table here.