I used to believe that on standardized test where it as "M" for male and "F" for female you were supposed to indicate who you are supposed to have sex with. So, if you're a girl you mark that you have sex with boys and if you're a boy you have sex with girls. This was before I knew about gay sex, which would have been even more confusing.
A childhood belief about sex

others believed things like this
- I used to think that to desex an animal meant to turn it from a male to a female or female to male.— Phil
- When I was a little girl, I believed that all cats were girls and all dogs were boys. :)— Mary Ellen
- I used to believe all dogs were male and all cats were female. I still call dogs "boy" and cats "girl." — Kelly F.
- I used to think when somebody sneezed you were supposed to say 'Gableshew'.— gablesh
- I used to think that when you had sex the male was sucking the life out of the female!— dude
- I used to believe all cats were female and all dogs were male.— Sabby