When I was eight, I sort of knew what "gay" meant. However, I could never figure out how two gay men could have sex. After thinking for a while, I concluded that the men would stand naked facing each other with their erect penises, and proceed to slap them together. In my mind's eye it would be a different version of a clapping game and they would sing a sort of rhyme to go along with the act.
I've been exposed to fanfiction and now know better.
A childhood belief about sex

others believed things like this
- I remember thinking that the "Nude Furniture" store was some sort of naughty place. — Texas Rocky
- I used to think gladiators were meant to be some sort of alligator.Anon
- I thought “guerrilla tactics” meant fighting like a gorilla— beating your chest and that sort of thing.— Kai
- I used to believe jaywalking meant walking around naked in public (from naked as a JAY bird...)— Izzi
- I used to think a clitoris was some sort of dinosaur.Anon
- I used to believe that women could only get pregnant once they were married and it just sort of happened Anon