It's one of those "to you it's a joke you just thought of, to them it's another reminder that people often don't accept them, either intentionally or unintentionally"

There are genuinely people who it would never cross their mind that two women without husbands living together long term means they might be married to each other or at least a couple. Some people will actively try and dismiss the fact that they're together and introduce them as friends. And others might just assume "roommates/friends" and ignore obvious "they're together" signs just because they don't get exposed to non-cis relationships often, not through malice or conscious effort.

I don't know how many times I've listened to a friend's parents introduce a girlfriend as "this is Xs good friend Y" only to have X correct "GIRLfriend. We're together." angrily.

There's a time and a place for all jokes, in my opinion. This just isn't it, I think. Just hits a sore spot.