@Steven: I’ve written at length on the subject of law. Take a look at that and I think that will answer most of your questions on my positions regarding law.
As far as how to distinguish law from personal morality, I think the distinguishing factor is the presence or absence of interpersonal conflict. If there is no interpersonal conflict, we are not really talking about law but individual morality. For example, let’s say Robinson Crusoe is stranded alone on an island. Giving in to feelings of fatalism, he simply lies down to die. A week later his body is discovered by some explorer who happened across the island. Aside from being unfortunate, we want to say there is something wrong with what Crusoe has done, that is, that simply laying down to die is reflective of a character flaw or moral deficiency within Crusoe. But, clearly, laying down to die on a deserted island out of a sense of fatalism should not be illegal because it doesn’t involve a conflict with anyone else.
If two adults choose to engage in illicit sex, we might question the individual moral problems that these individuals have but, clearly, it should not be illegal because they are doing what they are doing by mutual agreement. I think this is the mistake that some posters in this thread are making in thinking that there is no interpersonal conflict as long as the minor is consenting, so it’s merely a problem of individual morality. The fact is that there is interpersonal conflict between the guardian and the adult engaging in the predatory sex and that’s exactly how the matter would end up in a court to begin with.
As for your observations regarding the reasons why laws are what they are, I will simply note that the kinds of laws that are written down in the form “No person may _________” are not customary laws but statutory laws. See the linked article above for more explanation.
Clayton -