I too used to think that cats were girls and dogs were boys. We've mostly had female cats, so any male cat is known as an "honorary girl". When referring to them as a group, we always call them "the girls". With other animals, I used to think that the bigger animals were boys, and the smaller ones girls.
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