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.

So If you meet a couple and don’t know if they’re married or dating, you just decide to call them fiance/fiancee?

That’s like thinking “I can just replace null with 0 or empty string. Means the same thing”

"Fiance/fiancee" is not a middle-ground, it means you have proposed and your partner has agreed to marriage.

There are plenty of couples who may as well be married, but choose not to participate in that social construct.

Your comment about correctness is a bit ironic in context however.