In the UK I've run into people who use the term `partner' for whomever they are in a non-married exclusive relationship with, regardless of whether they are gay or straight. I don't know how common that is, or how it got started. I wonder whether it will ever catch on in the US?
It's common, but daft. It makes every relationship sound like a firm of solicitors. I don't see what's wrong with wife/husband/boyfriend/girlfriend.
"My significant other"