Here, too, easy examples can bring out the point. In the American South, before the civil war, slavery was legal. But despite the fact that slavery was legally permitted, we would certainly want to insist that slavery was morally forbidden: it was immoral to treat enslaved people as they were treated — indeed, it was morally reprehensible — and the truth of this claim is not altered by the fact that such treatment was permitted by the law. Similarly, to take a more contemporary example, opponents of abortion think that abortion is typically (or perhaps even always) morally forbidden — despite the fact that current law generally permits it. And if the law ever changes, so as to prohibit most (or all) abortions, there will doubtless be many other people who nonetheless will think that abortions are still typically (or perhaps even always) morally permissible — despite the fact that the law forbids it. In short, what the law says is one thing; and what morality says may well be another."

- Shelly Kagan, Normative Ethics (Westview Press: 1998), pp. 9-10 

See his conversational debate with Bill Craig here.