The story that introduced the character was drawn by Al Plastino and written by Otto Binder. Years earlier, Binder had co-created Mary Marvel, the sister of Captain Marvel.[4] Like Supergirl, Mary Marvel was a teenage female version of an adult male superhero, wearing a costume that was identical to the older character's other than substituting a short skirt for tight trousers. (Binder also created Marvel Comics' Miss America, a superhero who shared little other than the name with her sometime co-star Captain America.)

https://en.wikipedia.org/wiki/Supergirl

Its seems rather explicit - she was invented because the comic book creators wanted a femme superman - and so copy pasted the idea

Im not saying its right, im actually thinking that it is sexist and a product and fad of its era - and to erase it with retconned lore glosses over the very real human issues we face as a society that dimmenishes femme content as simply masc-in-drag content - im sure the modern movie is more timely and modern in its perspective - but to ignore the origins and further to insult the correct assumption i think is invalid