Oooh...just did some word searching.
"Betrothed," while accurate, is most commonly associated engagement/marriage. The other definitions get lost by the wayside, so it prolly won't work well.
"Plighted," while also accurate, is also torpedoed by the most commonly used meaning, so the intended meaning would get lost.
Now, "paramour" and "inamorata/inamorato" are accurate and widely understood (by those familiar with the words) in the fashion desired. They also sound much better (at least to me) than "girlfriend/boyfriend"--avoiding the negative connotations (in US culture, anyway) of "mistress" or "lover" (both of which have connotations of illicit affairs).
Plus, there's no need to coin new terms. Works for me. Work for anybody else?