Medullary bone does not let palaeontologists tell male dinosaurs from female ones — only laying females from everything else.

Palaeontologist and reader in zoology at Queen Mary University of London

So the instant assumption is, okay, so you can tell female from male. No, we can tell laying female from everything else. So males won't have medullary bone. Young females won't have them. Females outside of the breeding season won't have it.