Nicholas,
I think you over-simplify the safety issue. How safe is a transgender woman in a men's toilet? I'm guessing there's a way longer list of transgender women assaulted by men than women assaulted by transgender women.
Interesting study here of 98 transgender people, with 10% reporting having been physically assaulted in a public toilet because they are transgender, with one transexual woman reporting be sexually assaulted whilst using the men's toilet.
Then there are the practicalities. Let's say we adopt your solution. Who's going to police it? Some transgender women have penises, some don't. Will there be an inspector?
Like many things in life, there may not be a simple answer.
Who's to know they're about to transgress
if the plaque on the door shows a dress?
So I've ordered the signer
to design a vagina
it's the only way out of this mess.