The system doesn't need to be simplified quite that far before going to ridiculous.
Statutory rape and/or child abuse has a victim that is identifiable and distinct from the offender. The presumed harm done to the victim (and the class represented by the victim) goes so far beyond available civil remedies that the state pursues additional criminal punishment both to dissuade others from doing similar things, and to dissuade vigilantism and other extralegal retribution. We have a malum in se crime, an evil in itself. For such crimes, the state may put you in jail not just to make an example and to keep you from re-offending, but also to keep the minor's family from putting you into an unmarked shallow grave, at 2 AM during a new moon, with a strong false alibi.
The person who possesses, uses, or trades contraband offends only the state, which in a republic is not embodied in an individual, nor entirely distinct from the offender. Thus drug prohibition is a malum prohibitum crime, an evil just because someone said so. For the most part, no one cares if you smoke weed or not, unless you show up to work too stoned to do your own job, and then they only care because they might have to do some of your job in addition to their own. If you throw the offender in prison, that's a shame, because that guy is now doing 0% of their work, and their co-workers are now picking up 100% of the slack, until a replacement can be hired. If you don't go to prison, and don't miss work, the only ones you really harm are Philip Morris, RJ Reynolds, Jack Daniels, and Jim Beam, because the money you spend on weed cannot also be spent on tobacco and booze, and that is merely the sting from additional competition.