Sure. One is so idiotic that it is akin to saying "my head is fireproof!".
One could simply light their hair on fire to test it. Or, in the specific flat-earth case under discussion here, climb a reasonably tall hill see the earth's curvature -- no airplane required.
OTOH, there is in fact an empirical, science-based, opinion-not-required basis for the judgement of "male" or "female". (Even though, yes, there is also a tiny percentage of genetically anomalous cases that defy such classification, it's not germane.)
Additionally, though, there are centuries of societal reinforcement of various gender expectations, based on the inseparability of gender vs biological sex. These still manifest today in all sorts of ways, in traditions handed down from previous generations. Heard by kids from their parents, grandparents; reinforced in adulthood by all sorts of people.
Even though I mostly agree with your diagnosis of cognitive dissonance and "fringe" (I would call them "legacy") beliefs making this hard to accept, it is completely unsurprising that it takes more time for many people to process the upending of these definitions -- which in many ways are/were the bedrock of all sorts of societal classifications and expectations -- than it does for them to accept scientific truths established 500+ years ago, and which are anyway taught in grade school AND self-evident based on nominal and easily accessible experimentation.
Also, I don't think this is as much an issue of scientific (or moral) consensus as it is of semantics. Are you pro-choice, or anti-choice? Pro-life, or anti-life?
I think the side that wants gender to be immutably tied to biological sex (again, ignoring the actual biological anomalies) is wrong. It seems obvious to me, scientifically, ethically, culinarily, metaphysically, ... I mean, duh. But even though I personally don't have all that baggage like But what would dead Grandpa think? What would The Pope think? OK fine but what would the _previous_ Pope think?? it is obvious to me that for many if not most people in the world and the history of it, sex and gender roles are some of the most fundamental things.
So as we (as a society/species) tease out the difference between "gender" and "sex", I don't expect it to come as quickly and easily as the (extremely obvious) fact that the world is, in fact, not flat.
