I fully believed (before I learned learned that men had an unlimited supply of it until the vasectomy), that I only had a limited supply of semen. Mostly helped by the fact that I never had the same amount of it whenever I'd ejaculate. So I was always worried that I wouldn't have any left for when I was older and ready to have kids.
A childhood belief about rude bits

others believed things like this
- I used to think that in public toilets, the cleaning lady actually lived in the supply cupboard.Anon
- One day when I was 6 I learned that the toilet was NOT a water cooler. :/— J.C.
- so guess what i learned the other day: pickles came from cucumbers. yes indeed.— jenna
- We told our little sister that she had a long bushy tail and she fully believed us for ages!!Anon
- I fully believed, up until about 6 years old, that the word 'accident' was a noun that meant 'burnt toast'.— Karl Goetz
- I could never figure out what an "Elomenopee" was till I learned the alphabet a bit better. "L-M-N-O-P"— Lula