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.
As a cis by default person it is weird to me when I talk to people and they have strong feelings about their gender. But if we were normal we wouldn't be posting on this part of the internet