Use "friend." If your girlfriend, or whatever term you use, is going to get in a tizzy over something like that, I really don't think she has any business being poly. Being poly, for the time being, is not widely accepted. Part of the consequence in not giving a damn what society thinks and choosing a married partner is that you have to make some sacrifices (which, hey, I'm not at all against, mind you, in my opinion you should be free to love whoever you want). One of those is sucking it up and accepting the title of "friend" on auto insurance documents.
It sucks, and it's JUST things like that that drive me nuts. Try this: Ask your girlfriend to ask herself when things like this come up, is having that thing (whether it's being called "girlfriend" in public, or listed as "in a relationship" on Facebook) most important, or is it more important to be with him?
Whatever you do, don't list her as anything that MIGHT get you in legal trouble (like sister-in-law).
P.S. Or think of it like this, Does your insurance agent really give a damn about your personal life? Do you WANT them to be?