But Rob has a point, the English language (and many others too, German among them) is/are in dire need of a term for a non-married wife or husband. Like Rob, I think "girlfriend" sounds of schoolyard, "fiancee" is intrinsically a transitional term with a best-used-before-date, "significant other" is heartlessly stilted census form speak, "mistress" intentionally denigrating and "(life) partner" anodyne. Linguists would no doubt point to the fact that the lack of a good term is no coincidence: "Thou shall not (be able to properly) name, what shall not be." Yet times change (luckily so), language should catch up.
As a European I also squirm when I read about people past, say, age of 25, "dating" each other, that sounds to me of schoolyard too or of men in their 40ies/50ies in sports cars chasing women. I understand that it is ingrained in American culture, yet in German we don't even have a term for it! We use the English one since the last 30 years or so.