Non-binary option for characters, please?

[BillyKaplan]

Everything is essentially constructed by humans. The idea of time. The idea of gravity.

You're confusing 'defined' and 'constructed'. Time and Gravity both existed before humans ever evolved. You pick an item up, you let it go, it falls down. Gravity objectively exists because we don't float in space. Humans didn't construct it, we named it. We didn't invent time, though we defined units of measurements for it. Sex would fall under this as well (and even then there are exceptions to the standard male and female forms, and variations exist between species).
Gender does not. As is evident by what is considered feminine and masculine changes over time period and location. Pink used to be a masculine color. Guys used to wear high heels. Those things change.

Think of gender as color. For every color we have a clear and defined name. What do we call the sky and the sea? Blue. What do we call apples and sunsets? Red.

It's funny that's your example because we can't actually prove that two people see the same color. They just both know to call the ball they're seeing the color on by that name because it's a social agreement that that color, whatever it looks like to them, is "red". Regardless if what I see it as is closer to what you see as blue.
Another thing that's amusing in your example is that words by definition are context-driven. There are Eskimo tribes that have fifty words for snow. Because in their society, the nuances and contexts of snow matter much more than they do to people who live in other places and speak other languages. So you saying 'there is only male and female' isn't so much fact as it is a testament to the society and political climate you live in.

Nobody cares about your gender meat popsicle! Learn how to play your job instead.

"What's your gender?"
"Black mage"