The idea behind rumspringa is that being baptized into the Amish church is the start of a lifelong commitment, and not a decision to be taken lightly. It’s therefore thought that the person considering this step needs to know what they’re getting into, and what they’re giving up by doing so. And so, rumspringa.
The rumpspringa antics you see in the documentary “The Devil’s Playground” are real, of course, but they are the extreme end of the spectrum of what goes on during that time. The implied agreement of rumspringa is that the parents know that the kids will be checking out the English world, and my even stop dressing Amish or buy a car during that time. The expectation is that while they’ll bend the rules a little bit that they’ll still stick to the larger framework of the Amish belief system, so yes, sex with outsiders is strongly discouraged. Sex of any kind before marriage is strictly verboten.
Most of the Amish kids stay pretty close to the expectation. It’s more common for the boys to dress English than the girls, and there’s some drinking and smoking that goes on. But the rumpspringa of the typical Amish kid isn’t terribly exciting, which is why you see the extreme cases in “The Devil’s Playground”. That movie showed one Amish party where ~1500 kids showed up from several states, most of those kids were checking things out, maybe getting drunk, falling asleep in their cars, stuff like that. Most of them probably went back home after the weekend with the knowledge of what a hangover feels like and fell back into family life and pretended that nothing happened. And their parents looked the other way and did the same…