As far as I know it's got nothing to do with your body. You have to persuade the doctor that you are socially transitioned. What's the point in showing your body when it hasn't been medically changed?
And I don't think there is a law to say you have to use the toilet of your sex. Hence why women sometimes pop into the gents when there is a line.
Hence women not minding, in the old days, if a transsexual used their bathroom.
It's an understood necessity.
Paris Lees' assertion is counter-productive.
The reason bathrooms are separated by sex is because that's how everyone wants it. We are not legally forced to do it, everyone just likes it that way.