I found "Where Do Babies Come From" on our bookshelf when I was little, so naturally I read it because I was a huge bookworm. It said something about people usually having sex somewhere comfortable like a bed, so I thought that meant they had to be asleep to have sex. I believed this until I was 10 and my cousin turned on Cinemax when he was spending the night. I said "Aren't they supposed to be sleeping?" and he just laughed at me.
A childhood belief about sex

others believed things like this
- I used to believe that when people talked about 4x4 trucks, I thought it meant all the wheels turned.
— tracy - Before I knew about death, I used to think people turned back into babies again after they got old. Anon
- I believed that the pictures on my nighties came alive at night and played in my room while i was asleep!!Anon
- I used to believe that when people eloped that meant they were making babies...Anon
- I thought when adults talked about "sleeping with" someone they meant a sleepover!— Alberto Richardson
- I used to think a second cousin meant a cousin’s cousin.— Carlos Almonacid