Parents want to protect their children sometimes from the very mistakes they made at the same age, because they don’t want their children to have to go through the same thing.
Sex isn’t consequence free, there are STD’s, sometimes unwanted pregnancies in spite of birth control, sure you could say ok well there is abortion, but do you really want your kid to have to go through all that at a young age? You can’t control all their behavior, yeah they’re prob gonna have sex somewhere else if not at the home but you don’t have to enable the behavior either.
That said it probably depends on the kids and their maturity levels, it’s probably a case by case basis.