Use the sex listed on official documents?

So you want this guy in the ladies' room?

http://tinyurl.com/gomftct

And do you really think it's safe for this girl to use the men's room?

http://tinyurl.com/z6amznq

But are you planning on checking ID at the washroom door?  Or do you want everyone to drop their pants so they can pass inspection?  [BTW, that doesn't work because it is possible to change the gender marker on things like birth certificates and drivers licenses without surgery.]