Ignore naughty bits entirely! Everyone fixates on the naughty bits. Sheesh.
Imagine a 5'6" male. Imagine a 5'6" female. Imagine them wearing the same outfit. Do they look, stereotypically, different enough that you could tell which one is standing in front of you with high accuracy? Do you think this outward difference might be a useful identification tool to assess if a passport belongs to the passport holder? Must this indicator be sinister? Is it so sinister that the government should reject it for other, possibly more sinister indicators?