Circles are now squares
If a court ruled that Saddam Hussein killed Archduke Franz Ferdinand because some of the bullets he fired into the air during one of his speeches traveled back in time and space to 1914 Sarajevo, then everyone would rightly conclude that the ruling was nonsensical and invalid. Any public figure who attempted to uphold or enforce that ruling would deserve a long stay in a padded cell instead of plaudits for his loyalty to the court. If a legislature passed a law requiring courts to ignore bullets that were fired after the trigger was pulled and take into account only those bullets fired before the trigger was pulled, then this would not create an obligation on the part of judges who know the law of cause-and-effect to either enforce this positive law or resign. When the legislature asserts nonsense it is not the duty of sane judges to step aside and allow that nonsense to wreak havoc in the lives of the people whose cases the judge adjudicates (the same holds, of course, when the judiciary asserts nonsense.) To assert that it does is to reverse responsibility. Those responsible for nonsense are those who propagate nonsense. Those responsible for protecting the common good, whether they are judges or legislators or executives, have a duty not to resign and to resist it to the extent that the resistance does not become immoral.

Natural law deals with the nature of moral obligation. If a future course of action is immoral, then by definition it cannot be moral to carry out that course of action. Positive law cannot - it literally cannot - change this any more than it can change the law of cause and effect. Legal positivists might acknowledge the existence of the natural law (and everyone, in practice, believes in the natural law) but they'll deny that judges have the competence or the responsibility to take it into account in their rulings. Whether any judge is competent or not is a question of particular fact. But the notion that judges can avoid questions of natural law and the issues that arise when the positive law conflicts with it is clearly false. Judges can no more ignore the natural law than they can ignore the laws of physics.

Conversely, the notion that mystical "emanations and penumbras" emerging unseen from the positive law create moral obligations on the part of the citizenry is equally false. It's as false as believing that courts can alter the laws of physics in declaring that Saddam Hussein started World War I. Positivists might think that natural law and penumbral lawa is a distinction without a difference, and that we can avoid questions of what is true by sticking to the positive law alone. The difference, though, is unavoidable. Courts literally cannot alter the laws of physics and create facts which are false. Likewise, courts literally cannot alter the natural law and either impose a moral obligation to do something immoral, or make an immoral act moral. The issue of what is true cannot be covered up with any amount of procedural wallpaper. Otherwise, we're all postmoderns in a world where circles can be squares.

EDIT: Andrew Sullivan is jubilant. He asks rhetorically: "So the initiative question becomes: do you want to divorce thousands of already-married couples? Or do you want to keep things as they now are?" That's what I was talking about. If a state constitutional amendment was passed then it wouldn't be "divorcing thousands of couples" because those couples were never really married. Likewise, if I were a business owner then I wouldn't extend benefits to same-sex couples like I would to married couples, because the latter are married and the former are not. Call it discriminatory if you wish, because I am discriminating between fantasy and reality. And I'd cheerfully go to jail before I recognized that falsehood as reality.