When I was about six, I asked my Mom where babies come from. She gave me a reasonably detailed explanation, including how sex works. Except she never explained that the guy's ... um ... penis has to be ... well ... erect. So I used to believe that sex involved trying to get an essentially flaccid and floppy object into a small space. Kind of like threading a needle!
A childhood belief about sex

others believed things like this
- I used to believe that gay sex involved a man sticking his penis inside another man's penis.Anon
- I used to believe that babies came from outer-space. :)Anon
- I used to believe that babies came out of their mother's mouth like a small bubble.— Evelyn
- When I was young my mother told me that old people kind of shrink. I thought that they shrunk into babies.— dont know
- when i was a little girl i asked santa for a penis for christmas. i thought it was a really awesome toy. — haha
- i used to think that vacuum cleaners had little piece of space inside them because space is a vacuumAnon