"Partner" - now that really is an ambiguous term. Even "Sleeping Partner".

I think "spouse" is a word that deserves to be brought back into use. And it's got the advantage that it could be used by gay couple in "civil partnerships" just as well just as well as people in (mixed) marriages. And there'd be no need for any legal shenanigans in using it, and no legal ground for anyone to challenge it so far as I can see.