I had a brief phase of thinking that sharing a water glass with a man would get you pregnant. (I think I knew it could only be grownup men, so it was ok to share with my brothers.)
In 5th grade sex ed, they explained everything in a fair amount of detail -- the menstrual cycle, anatomy (I think of both girls and boys), how the egg and sperm get together and fertilization happens. The one thing they left out was precisely how the egg and sperm could arrive in the same place. (We got that in 8th grade sex ed, but by then I already knew.)
5th grade sex ed was not co-ed. I assume the boys got less information about menstruation, but nobody asked. The boys did report that they watched a film of some sort in which the egg wore an evening gown and the sperm wore a tuxedo and they had a night on the town or something. I wonder how many boys were confused by that?
A childhood belief about getting pregnant

others believed things like this
- I never knew why boys had to lift up the toilet seat...until my friend told me that boys had bad aim.Anon
- when I was a kid, I used to believe that only girls could be my friends and all other boys were my brothers.
— rajat - I thought sperm were fish and when I got a goldfish I went around telling everyone i had a sperm— Kara
- I used to believe the Beastie Boys were a boy band. Well, the name sounds similar to Backstreet Boys!Anon
- that cats were only girls and dogs were only boys. I definetly didn't get the whole reproducing thing yet.— Jamie
- I used to believe that boys, gave birth to boys and girls gave birth to girls. How i'm so wrong now.— Sarah