While I always knew the physical differences between boys and girls, until age 9 or so I thought that penises/testicles were strictly to distinguish boys from girls when they were born.
A childhood belief about rude bits

others believed things like this
- I used to believe both boys and girls had penises so i thought sex was when they tied them together.Anon
- I thought that the difference between girls and boys was that girls have teeth.
Anon - i used to think boys were born from men and girls were born from women— swag lad123
- I used to believe all babies came out as girls and later on in life some of those girls turned into boys — Tom
- I used to believe that boys, gave birth to boys and girls gave birth to girls. How i'm so wrong now.— Sarah
- I always thought when I was a kid that Milo is for boys and that milk are for girls. . . — Cathyia