common law fiance
When a couple has been "dating" for such a long time that they are on an inevitable path to marriage, the participants in the relationship become common law fiancés, even if no formal commitment to get married has been made. Often, the involved parties deny this de facto status, frequently out of fear of commitment or embarrassment over not taking their relationship to the next level. However, they usually conduct themselves like a typical engaged couple (e.g., sharing holidays with both sets of families, etc.)
Aaron: hey, is your fiance coming to dinner with us?
Mike: no, my girlfriend cannot make dinner tonight
Aaron: she is so your common law fiance...you guys have been together for more than 6 years!
Mike: no, my girlfriend cannot make dinner tonight
Aaron: she is so your common law fiance...you guys have been together for more than 6 years!