Not sure I get it, is this the meme thing where they're slamming liberals for some obviously bad outcome? I mean, that would be hilarious to realize how annoying that phrase might be to the people who would be bothered by it, but aside from that it's hardly a 'bad outcome'.
Or maybe the meme is being used ironically. is there even a way to tell?
Whenever I see the phrase (on tumblr, on reddit, on the web in general), it is generally being used by liberals to highlight something that we think conservatives would wildly disapprove of and liberals would applaud.
I've posted some images lower down (or higher up depending how you sort the thread I guess) about it, and don't want to post them again, but they give you the general idea. Mostly -- when I see it -- the phrase is used by liberals to mock conservatives about their views about how open and free the liberals think the world will be, and how truly appalling that will be and how sad and terrible a place it will become.
So in this instance (and now that I am explaining this you get getting way more of a look into my head than I ever thought anyone would) my implication is that conservatives have set up a situation where if Harris wins, the term "First Female President of the USA" will refer to a black lady by default.
This generally isn't the case in most western countries. (Sad, but true). When a "first female" something happens (Prime Minister, Leader of The House of Commons, Director of The Met Orchestra, head of a union, head of the space program, head of anything -- the first woman in any of these posts is generally white. This is true in the UK, and it is almost certainly true in America. (Unless the post is "head of an organisation specifically aimed at black people" then there's usually an exception)
This is why when most "First woman" to hold a role (Prime Minister, Head of The Met, Lead Conductor of The Met etc) their race isn't mentioned at all because no one thinks twice about it.
But if Harris becomes the First Female President, then the Second Female President (if she is white) would have to be noted as "The First Female White President" or just "The Second Female President"
It is just something that some people find interesting, and kind of funny.
It is also something that a lot of people wouldn't think about, because the missing word in "First Female" would never occur to them.