Yup, I've been in my (heterosexual but not married) relationship for long enough that the word "girlfriend" doesn't really feel like a good descriptor anymore.

Boyfriend/girlfriend felt like a bit of a weird way to talk about each other when we were both in our 30s. And then fiancee was an awkward word. Maybe we should have used partner

I use it all the time, who is making this assumption?

Mine absolutely adores messing with that assumption. I'm nonbinary and she's bi though strongly prefers women and more ambiguous to fem leaning nonbinary people. She looks like a very Butch lesbian but together our relationship is straight passing since I'm built like a fridge cosplaying Harry and the Hendersons and when my joint pain acts up like the most tips fedora Neckbearded MF to ever haunt the halls of an anime convention.

So when she's alone and everyone has her pegged dead to rights as a textbook carpet muncher she plays into it going "My partner this, my partner that," just to see the look on their faces when my literal grizzly bear ass shows up with our 3 kids or she decides to switch it up to mention her husband and kids.

it is hilarious.

That is due to frequency of use in past decades carrying over as a habit. All the gay people I've known in the US used partner in the 90s through whenever we finally legalized marriage (2015?) because they weren't legal.

Those same people tend to still use partner, because old habits are hard to break.