I have to admit that I've only recently realised that this one was a thing, and it's all because of this video. If you haven't watched this already, take a good look.
It's surprising often this turns up, isn't it. Sexy woman emerges, fully formed, but is also a blank slate for the male lead to impress upon. Everything he finds good, she meekly follows along with and finds good.
Keep in mind that he is in control of all of her worldly experiences. He's her educator, her filter through which she sees the world, and, creepily, something of a parent as well.
I know, right? Euw.
More recent editions of the BSY girl have had her be magically spectacular at something that males find respectable. Like awesome sexy combat skills. Or awesome sexy car repair. Or awesome sexy kung fu pole dancing.
Okay, I made the last one up, but give them time. It'll happen.
The key point is, even though she's awesome in a limited and very specific capacity, she's an innocent, childlike, blank slate for the hero to shape to suit himself.
And it's a trope that goes as far back as Pygmalion.
The easiest way to kill it is to write three-dimensional female characters and male characters who can appreciate that. Most of our social education comes through our media and our media is, frankly, a little bit scummy when it comes to relationship portrayal.
So the next time you see someone who was Born Sexy Yesterday, talk about how rapey and pedo-y that entire thing is. Shine the light on how really awful that kind of relationship could be for both parties.
And don't shut up about how to fix it. That would be fabulous.