I've been a grown adult for a long time and still use boyfriend/girlfriend. This obsession with absolute correctness is probably why we are so miserable all the time.
To me it's simple
1. Married -> Husband/Wife
2. Dating -> Girlfriend/Boyfriend
3. Middle Ground -> Fiance/fiancee
I've yet to see a case this doesn't cover so "partner" seems like a solution looking for a problem.