In the FB discussions I have seen, this has not been a problem in practice for anyone who has gone through it.
The key to keep it within legality is to use your legal sex marker on the reservation / ticket.
If, like me, you can be "read" at a glance, then oddly that can be easier than if someone really has to look hard to see your birth sex. If "no-one is fooled" then you are not "passing yourself off" as the other gender.
