DH was from a family of brothers, and although they all discussed sex, they didn't know a thing about periods. When I first discussed it with him, he said he knew girls had bleeding once a month - but he thought they sat on the loo when it started, and it all gushed out, then that was it - over! He was quite shocked when I told him it lasted for days
I don't think it's that unusual. I read something about a debate about providing free or subsidised sanitary products to those who need them and a man seemed baffled about why it was necessary and if they weren't so lazy they could just hold it in, like urine, until they next went to the toilet.
Anyway my own admissions...
We went on holiday to Scotland when I was 6 or 7 and our parents told us that haggises were real creatures. They bought us some fluffy toy haggises from a toy shop and we assumed that was what they looked like. When we were walking in a forest next day I was sure I had seen a furry haggis run across the path! Many years later when I was mid teens we went to Scotland again. One day my parents were laughing uproariously about how we'd once believed in real live haggises as small children. I laughed along, only realising with horror at that moment that I'd still believed it.
Also when I was first told the facts of life around the age of 9ish it was only the very basics about PIV. Nothing about erections or ejaculation. I thought the man just stuffed his floppy willy in and the couple then just rolled around kissing for a couple of minutes (that's what you saw on programmes like Dynasty etc. ) It seemed incredibly dull and I couldn't understand the attraction!
Also - air strikes - I thought it meant no planes were flying because the pilots were on strike.
Oh and when I bought my first property in my early 20s and the heating element must have gone kaput, I had no idea about such things and I rang Thames Water to complain that the water coming out of the hot tap wasn't ... hot. I'm mortified to remember that.
I'm actually quite clever believe it or not.