Sherlock remembers the first time he passed. He's pretty sure everyone like him does. He was flying to France with Mummy and Mycroft and had just cut his hair. The passport photo had been taken a month before, when his hair was longer. Sherlock remembers the thump he felt when he heard the woman at customs: "Where's the passport for the other boy?"
It lasted only a moment, of course. Mummy flushed, laughed slightly, and brought him closer so the woman could see his face and associate it with the bold 'F' on the passport. She retold the incident, later, laughing that her daughter had been mistaken for a boy.
Sherlock looked at Mycroft, and grinned, wide, knowing they were thinking the same thing: the woman had not mistaken him for a boy. She could tell he was a boy.
The grin did not leave his face for a week.
A/N: The term 'passing', used by Sherlock in the first line, is controversial. It carries connotations of deception - you are tricking people into thinking you are something you're not. The whole point, of course, is that people are seeing who you are. Unfortunately, the primary alternative is 'successfully presenting as male/female'. I normally say that, but I can't see Sherlock putting up with cumbersomeness of terminology for the sake of less awkward connotation. Hence 'passing'.