@TeaPoweredScientist
Or perhaps the onus is on us to "prove" our womanhood, if our word isn't enough? So, should we start carrying around copies of our birth certificates so we can show them to anyone who challenges our presence? That's dystopian as all hell, and also reveals my (rather uncommon) full name to a potentially-homophobic stranger who might use this information to stalk, threaten or harass me.
That's exactly what some gender critical groups want, as Fair Play For Women explain here: fairplayforwomen.com/changing_rooms/
"It’s a well established custom in the UK that people use the changing rooms appropriate to their sex. However, in order to uphold your lawful policy of providing women with a female-only changing room it may sometimes be necessary for staff to deny access to a male person. We instinctively know who’s male and female simply by looking which makes it quick and easy for staff to spot if a male person enters a female changing room.
To uphold your single-sex changing room policy you can reserve the right for staff to discreetly ask for additional information before using the service you are providing. The only legal document that confirms someones legal sex is a birth certificate. All other common forms of ID, including passport and driving licence, can now be updated on request to show someone’s self-declared gender identity so can no longer be used for the purposes of confirming legal sex class."