It's not about being dense, it's about not seeing them as a woman.
Like me personally had a friend in highschool who dropped lots of hints, and I tried picking up on them after she broked up with one of my friends, but then she shied away, and gave a bs line where she "brother-zoned" me. Don't say shit like, "That girl you're dating is too dainty, she'll break if you two slept together, if it was me I'd be able to take you" fucking bullshit
Fast forward 3 years or so, she starts dropping hints again, dressing up with sexy stockings and showing tits around me, but I couldn't see her as anything but a younger sister after being bro zoned.