This is quite an old thread but I'll bump since we were talking about this with my girlfriend just yesterday.
Well, more accurately we debated whether she should/could call me wife once we're that far. It's only been 2,5 months so not for some years, but we enjoy talking about this kind of stuff.
She said she would feel as though she was misleading people, since we are both already married and thus can't marry each other (if we were single we could get a legal partnership in our country despite both being women). But I feel as though I'll have every right to call her wife once I feel ready to get married. Society won't recorgnise the commitment for each other, but I want the word to reflect what I feel and our relationship. Actually, I feel it as a political thing to do signifying that society should
recognise multiple relationships legally.
If one wanted to be even more political, they could adopt "Unlawful Wife" or something like that.
But I find wife to be more practical in everyday life.