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.
A lot of people question it and just go "this is well enough for me". I've wondered about it plenty butI think for my (completely personal) purposes it would be hair-splitting at best to object to being called a guy because my body is my body and society is organized in a gender binary. I despise the social construction of gender, but I also dislike the English language and yet here I am using it to participate in society in a relatively frictionless way, and for me personally it's kind of the same thing.
I can totally see finding it weird as a nonbinary person how people can feel fine as a binary gender, cis or trans.