[She's quiet for a moment because this is too important a decision to be reacting on pure instinct. When she does finally come up with the right words Nariko lays a hand on his shoulder very carefully, mindful of the possibility that he might not want to be touched.]

Do you ... Want to be with just one person? It's a considerable change for you too, right? I'd understand if you'd rather things stay the way they are. [She actually doesn't, in the strictest sense, polyamory is a difficult concept for her to grasp, but open relationships aren't, and her feelings for Boxer are pretty clear by now - she's pretty much in love with him, enough to understand something difficult for both their sakes.]

I don't want you to feel as if you have to change something you enjoy to keep me around, that's not what this is.