Up until the later years of high school I thought that "oral sex" was making-out and that you couldn't get STDs from it unless someone had a cut in their mouth.
(though, in my own defense, this IS what my junior high health teacher implied)
A childhood belief about oral sex

others believed things like this
- Until junior high school, I thought Monty Python was a wrestler.— Jamie
- I thought that wars had to last four years... just like high school and college.— A. Falk
- My teacher held out a hand to say, "stop" but I thought she wanted a high-five.Anon
- Until I was in High School, I really thought that the "underground railroad" was under the ground.— S
- I used to believe that you got married right after you finished high school or college. Anon
- When I was young, I honestly believed that I would die before I reached high school. Anon