It's the classic puritanical/quaker view of sexuality. Kids should be shielded and protected from any mention of sex and sexuality. Then when they turn 18 or get into their first sexual situations a switch is flipped and they're chastised for not "knowing better".

Why are kids special?

I think of that quote:

“The unborn” are a convenient group of people to advocate for. They never make demands of you; they are morally uncomplicated, unlike the incarcerated, addicted, or the chronically poor; they don’t resent your condescension or complain that you are not politically correct; unlike widows, they don’t ask you to question patriarchy; unlike orphans, they don’t need money, education, or childcare; unlike aliens, they don’t bring all that racial, cultural, and religious baggage that you dislike; they allow you to feel good about yourself without any work at creating or maintaining relationships; and when they are born, you can forget about them, because they cease to be unborn.

Children aren't quite as good as the unborn, but they're close. Advocating for children still lets you feel good about yourself without having to actually associate with children. They're a group it's pretty much OK to be paternalistic toward. If they do resent your condescension, you can easily write it off because they're just children.

I answer every question my son has honestly and in plain, easily understood language. He's in middle school. Boy are his friends being fed some bullshit from their parents and unfettered access to the Internet. Smile and nod, kiddo, then come ask me. If I don't know, we'll look it up together, but I will not keep him in the dark. The dangerous thing about sex is ignorance of it. And yeah, porn ain't real.

So glad you brought it up. Not only sex but romantic partnership in general seems to be the one thing people need to somehow figure out on their own.