As I've probably mentioned before, the law is hampered by our judicial system.
Because jurys and judges can be swayed by evil, money-grabbing, b*stard lawyers (did I mention I don't like lawyers? ), many laws have to put black and white limitations on what should really be morally "grey" areas, so that convictions can be made.
For example, the "underage sex" thing. Many people are sufficiently mature to have consenting, safe sex whilst they are still considered to be legally underage. Whereas many people
considered legally old enough are nowhere near emotionally mature enough to have a sexual relationship.
Morally, it's a grey area and would ideally be treated on a case-by-case basis, but in order for the judicial system to work efficiently, black-and-white lines have to be drawn. In most cases, these lines are in the right place for most people, but an unlucky few WILL get caught out where their personal (and possibly valid) morals differ from those of law.
As a further illustration; the age of consent differs quite a bit throughout the world (http://www.avert.org/aofconsent.htm). Are we sure that ours is "correct"?