For a short time between ignorance and fuller knowledge (I'd guess about 10 to 12, maybe younger), I knew that girls did not have penises. From there I concluded that boys had sex with girls by putting their penises into girls' asses. The confirmation of this theory was that girls had cute butts.
A childhood belief about sex

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 believe all babies came out as girls and later on in life some of those girls turned into boys — Tom
- Since the girls name Jackie is short for Jacqueline, I thought the boys name Jack was short for JackalAnon
- I used to believe that boys, gave birth to boys and girls gave birth to girls. How i'm so wrong now.— Sarah
- I used to think the girls' name "Tina" was short for "Argentina".— Kristen