That a major portion of their self-image/identity which is gender, is more in their head than they'd like to think. Whenever you upset a notion about identity it comes with a harsh reaction to those who have not their house or life savings but their self invested in that notion.
Religion, nationality, race, gender, age, creed. These ordinary words on their own are big ominous headings for topics on social issues. Mess around with any on of them and the congregation of identity subscribers will take a dump on your pillow.