The Egyptians had the Ogdoad, four male-female pairs: Nu and Naunet, Amun and Amaunet, Kuk and Kauket, and Huh and Hauhet.
Neith (also from Egyptian mythology) was said to be an androgyne although she was usually depicted as female.
It's not so much that we don't understand it. It's that we don't understand it yet. We haven't had our brains dissected yet (I don't think) and haven't been researched to the same extent that binary people have (if at all). Therefore, we don't know what they look like yet.
Personally, I've found that just being myself and letting other people figure it out doesn't work at all. People are too busy telling me who I should be and who they think I am instead actually bothering to see me.