Oh, that's one of the creative reasons, yeah.
The Sisters of Battle also lend into a lot of the cheesy "action woman" tropes in the 70's and 80's, too -- possibly with a bit of the likes of Red Sonya and Wonder Woman thrown in.
If a woman went through the augmentations somehow (let's just say by ROB), they'd be biologically superior to a baseline man, yeah... but to other male Firstborns or Primaris that went through the same processes? Yeah, you'd basically have an inferior copy because a man's biology is built for fighting (hearts being larger, denser musculature, et cetera), and that's what the augmentations themselves are built on.
Unless they repurposed these discount female "Space Marines" for another purpose, like making them assassins (but, again, there are already multiple assassin orders in the Imperium) or disposable troops compared to a normal Marine's usefulness (and wouldn't that set the weirdos off? Women being used as suicide troops, haha), but then again you might as well just create a whole slew of new augmentations and shit instead of repurposing the Firstborn and Primaris procedures.
So, yeah. Female Space Marines? Not happening creatively or in-universe, and feminists, soy boys, and transtrenders can whine and cry about it all they want.
Edit: Ironically, a transgender (MtF) would still be a candidate for the Firstborn or Primaris augmentations because they're still biologically male, but I get the feeling that isn't what the weirdos want, haha.