When he was a kid, my friend (who's gay) noticed that cats only mate with cats (and all cats are female) and dogs only mate with dogs (who are all male), so, of course, it was logical to him that, in humans, boys should only "mate" with boys and girls with girls. Makes sense to me! :-)
A childhood belief about sex

others believed things like this
- I use to believe that cats and dogs were the same species; cats were girls and dogs were boys.— poopcrystals
- My sister and I thought cats and dogs were the same animal, the dogs were male and the cats were female.— Paul
- I used to believe that all cats were girls and all dogs were boys.— jordan
- I used to think that all cats were girls and all dogs were boys. — trav
- I used to believe all dogs were boys and all cats were girls.— Liz
- I used to believe that all cats were girls and all dogs were boys. Anon