[Serious] Please convince me gender is binary

[The Grand Nudger]

I'm pretty sure that no one expressing a non heteronormative position is talking about their genitalia.

No, but they are talking about the kinds of roles and attitudes that are typically found lined up pretty neatly with genitalia. 

If, by line up neatly, we mean lines up neatly with whatever gender normative social construct exists in a specific society..then yeah, sure, because there isn't a specific attribute or trait that lines up neatly with genitalia as a broad rule.

If gender is whatever one is, then how could you have a trans woman?  What, exactly, would such a biological man be transitioning to?  I'll tell you what-- he's transitioning to the behaviors and fashion sense that is normally associated with people born with vaginas and XX chromosomes.

You're still talking gentalia, but gender is only coincidentally related to genitalia.  A trans person is perfectly capable of holding heteronormative gender constructs.  Most are expected to, regardless of the contents of that normative construct.  

There are actual women, and there are biological males who act in ways that women generally act.  To pretend there are no norms, no general trends that are defined by physical phenotype, is to ignore the reality of the human species.

No one pretends that there are no cultural norms, no one.  The only reality of the human species being discussed by gender conformity or non conformity is the reality of gender constructs.  There is no specific, species wide reality of gender whatsoever, regardless of the fact that we do or don't have a specific set of genitalia.   

Don't believe me?  Define "woman" at all.  In any regard, under any position heteronormative or otherwise.  What's a woman?  Anyone who cares to identify using that word?  Can Bubba with a beard and a donkey dong put on a little mascara and make us call him missis?

A woman is whatever the culture in question deems it to be. The answer to that question "what is a woman and what makes a woman a woman" is an explicitly cultural question with an explicitly cultural answer. It is not the same question as "what genitalia does a person possess".

As I said before....when a person is telling you/us that they're non binary or non normative...they aren't making some comment on the presence or absence of penis..nor would you/me reaching under their skirt add or subtract anything to or from what they're discussing. These people know full well what sort of plumbing they're packing...and they understand that culture has placed expectations of behavior and normative stereotypes on that plumbing.

This..is exactly what they're discussing..... The failure of our binary heteronormative constructs to justify themselves or the consequences of our holding them.