Armor designs for females?

[Keltest]

And yet for some reason this argument only comes up about "sexy" armor, not the thousands if not millions of characters who go into battle unarmored without anyone blinking an eye.

As I said above, there are many genres, both historical and fantastic, where armor simply isn't a thing despite an abundance of melee combat.

Depending on the circumstances involved, that could be for a lot of reasons. Enemy weapons invalidating armor, where in the world you are, who the combatants are, quality of materials available for armor and weapon construction, magic... all of those can affect what armor, if any, is being worn.