Note: Lots of diffrent legends for all the Nations!
Just So Stories
Why the Male is Called "Son"
Once, a very long time ago, there was no sun. Only the cold blackness of night, lasting forever. This was a time before the moon, a time before any bending. Life was void, everlasting, empty, filled with fickle things of no meaning.
There was man. Before the moon and the sun there was man, but these people where just shells, for souls could not exist with out bending, without spirit or belief. It is said that is why the Great Spirits sent the moon. They sent it to show they were all powerful, and they alone could create and destroy.
But the moon alone could not shine. It created the water benders, but the world was so completely and utterly off balance that the Spirits had to create something else to counter the water, but at the same time help it.
So they created the sun. It only seemed fair to make it of fire, because air had the sky, water had the ocean and moon, and earth had the soil. Combining all their power the Great Spirits formed it into a sphere, and called it "sun".
It was the biggest of the Spirit's creations, and they believed the most wonderful. Soon the Great Spirits valued the sun more than any other creation, and wanted humans to mirror its greatness.
These humans were called fire benders. For a time everything seemed at peace on the world, but when one child is favored over another, all balance is lost, and chaos replaces it.
The moon, the air, the soil were all jealous of the sun. When jealously erupted, so did hate, and killing, and pain, and death. And war. Because of the sun, the fire benders believed that theirs was the superior element.
Then rivalry came, the kind you get between siblings, children. It was childish, yet just as full, and caused the water to forget its healing, the earth to forget its growth, the air to forget its breath and be replaced with horrid things; drowning, crushing, and suffocation, because the fire had long ago forgotten it's warmth and only burned.
So the hatred was there, as old as the sun, but a war didn't result until many centuries later. Even so, it still became a sort of sick tradition (though most forgot its underline meaning) to name man, the superior being, after the after the superior elemental creation.
And to this day, when man has a child who is male, he is honorably called "son".