...when was the last time you got asked for your ID (still less your birth certificate) to go into a public lavatory? If you do start asking people to prove their gender before entering a single-sex public space...

One of my worries is that they'll try and close loopholes around this by requiring a gender recognition certificate to change the gender on a passport or driving licence, which will fuck over a whole load of people (including me, but others will have it worse).