Who exactly are you to say what is or isn't appropriate for elementary school libraries?
It's authoritarian because it's about people with authority (parents, teachers), telling people without (students) what kinds of media they are and aren't allowed to consume, which is about controlling which ideas they're allowed to think about. You don't like children thinking or learning about sex, but there is no moral or rational reason for that. You just don't like it, and you wish to use your authority to impose your preferences on people who have no power to stop you. That's authoritarian.
And no, I don't think parents should be able to control their children's media diets, the idea that parents get to control their children is itself authoritarian. You don't own your kids.