This is actually a friend of mine. Somehow or another as a child she came to the belief that all dogs were boys (male) and all cats were girls (female). Even after a sexual education class she never questioned this until the age of 19 when someone asked her how they could reproduce if each species had just one sex.
A childhood belief about cats and dogs

you weren't the only one
215 of you believed this
- I used to believe that cats were all female and dogs were all male.— Rachel
- 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 a dog was the male version of a cat.Anon
- I believed that all cats were female and all dogs were male. — Biologically confused
- i thought cats were girls and dogs were boys. Anon
- i used to believe that all cats were female and dogs were male!Anon