My two older sons are at a psychological stage which involves extreme alliegance to their boyness and avoidance of anything remotely girly. Our house is infested with weaponry, dinosaurs, superheroes, and books on snakes. These days I'm starting to read them from the DK series of biology-related books. I got them hooked with the danger end-- insects, poisonous animals, etcetera-- but we have also started into microscopic life. One wrinkle has been that the textbooks are written in German. I didn't know I was reading about "Reproduction," for example, until we reached a blithe discussion of a molluscan chain fuck, and I had to start furtively paging ahead to see where exactly this text was headed. (Anatomically exact, cutaway depictions of fly-on-fly action. But mammalian fun 'n' games was limited to a pretty modestly photographed pair of cats)
Their boy ID is so strong that we actually got into a disagreement about the heroine of a chapterbook. Both sons were certain she was a boy-- I think they literally cannot concieve of a girl liking kickball-- and I kept saying, no, Junie B is a girl, just look at her name. But this series has been written with relatively infrequent use of gender pronouns, so I couldn't seal my argument.
The issue was finally settled when we arrived at an illustration. Oh, she must be a girl, Matthew said, upon inspecting the picture. She's got flowers on her shirt.