@PurpleCiitrus From the manga's context, they never met a boy, not even once.
All they were told about it was that boys are dangerous and they weren't taught anything about their appearance or how exactly are they different from them(girls).
And if you are curious about it in real life.
Then it would be for young girls. Either because they don't have a brother or they were able to avoid this question for a long time.
Or they just assume that it's same as theirs (child's fantasy).