Control, mostly, at least at the meta level.

Many of the parents who freak out hold a lot of shame themselves surrounding bodies & sex, having been denied good sex ed. It is their job to ensure they don’t pass that to their children, but I imagine this is easier said than done, especially if shame & ignorance are normalised within a culture.

For them, the best argument may be that children who do not know about sex are vastly more vulnerable, not only to bizarre information and unpleasant mistakes, but to sexual predators.