Where it will get interesting is scenarios where a biological male is recorded as "female" on a work employment system and is acting in "stealth" at work. I have a real world example of this but it relates to school residential accommodation instead of toilet facilities. Although it relates to accessing a different single sex space (in this case, going into the girls' sleeping accommodation on school trips), the issue at hand is exactly the same. How can employees stop biological males from accessing female spaces if they insist that they are female and there is no written evidence that they are not?
In my IRL example, there is a teacher who is very obviously male, from observing head shape, shoulder shape and gait. I've never heard this teacher speak but the other features are so obviously male (the gait being the biggest giveaway) that I really don't need any more evidence to trust my own eyes. I've not specifically named this teacher to staff but I found myself in a conversation with two staff members when it was obvious that they knew exactly who I was talking about (I assume because they also have eyes). One staff member said "I think I know who you are talking about and you're wrong". This teacher goes on school trips with students. AFAIK this will involve going into the girls' accommodation.
Obviously I could be wrong about this teacher's sex. But I really don't believe I am. I suspect instead that this teacher has done a very good job of getting lots of sympathy for "often being mistaken for a man". I should also add that on some occasions, this middle-aged teacher goes to school wearing long stripy socks. I've not seen that myself but my daughter and her friend told me about it: they were talking about how oddly this teacher dressed sometimes so I asked for an example (as an open question) and this was it.
It should be simple for employers to keep biological males out of female spaces (toilets, changing rooms, sleeping accommodation etc) but I would love to know more about how they can in scenarios where males can still both deny and legally obfuscate their sex.