For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some 'organic element' since I couldn't accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
It wouldn't surprise me one bit, if it turned out that the most common gender ID is actually non-binary.
Like, my spouse and I pretty much consider ourselves "cis" but ... but not for any particular reason other than, "well, its good enough to get the point across." We don't really "feel" any particularly strong emotions about it.