Well, I'm going to contradict myself almost immediately, and say there is a tricky situation already arising, which Helen Joyce's observation doesn't help with. That's one where a trans person is out to their boss but not their colleagues, meaning the boss is now obliged to formally instruct them to do something which will 'out' them. If boss then turns a blind eye to continued wrong-sex toilet use (which might seem like a pragmatic, albeit unacknowledged, solution) they will either be deceiving their other employees or, more likely, forcing them to continue putting up with something they were already unhappy with, but couldn't say so (and still can't, without trans-hunt finger-pointing).
(I know people in HR grappling with this.)
Long term solution, obviously, is for everyone's sex to be known to all.