P1. Not everything morally impermissible should be legally impermissible.
P2. Not everything morally obligatory should be legally obligatory.
P3. Not everything morally permissible should be legally permissible.
Ad (P1). This could be put idiomatically by saying that not everything morally wrong should be illegal. I hope you will agree with me that drinking oneself into a stupor is morally wrong. It's a Friday night, you are alone, you stay in your house the entire time, but you down a quart of whiskey. I hope you will agree with me that this is morally wrong. (I could give an argument, but it belongs in a separate post.) Though morally wrong, such behavior should not be illegal. If you don't like my example, you are invited to concoct one of your liking that makes the same point.
Why should not everything morally wrong be illegal? The positive law is made by human beings and enforced by state power. But it is notorious that human beings are to varying degrees morally corrupt and that they are therefore strongly tempted to misuse any power that they acquire. Note that I do not say that power tends to corrupt its possessors, but that our antecedent corruption inclines us to the misuse of such power as we possess. (Lord Acton take note.) This misuse of power is especially clear in the case of the agents of the state. (Ever have a run-in with a cop, a sheriff, an IRS agent who overstepped the bounds of his legitimate authority?)
Given the egregious violations of individual liberty including mass murder that state agents have been known to commit, reasonable people support limited government. One way to limit government is to restrict its sanctions to behavior that impinges upon public welfare. Thus there needs to be laws against drunk driving. But laws against private drunkenness (as in my example above) tend to give too much power to the state, besides being unenforceable. Laws that are unenforced, however, tend to breed disrespect for law. Without widespread respect for the rule of law, all the laws in the world will not contribute to public well-being.
Laws should be few in number, rational in content, understandable by the average citizen, enforceable, and enforced with credible sanctions. So not everything morally wrong should be illegal.
Ad (P2). If you morally must do (ought to do, are obligated to do, have a duty to do) X, should the positive law require it of you? I answer in the negative. Consider the philosophical/religious duty to worship no such 'false gods' as money, power, sexual pleasure, Lady Luck, but to worship or devote yourself only to worthy objects such as God, truth, justice, benevolence, and the like. I would argue that it is morally impermissible to worship any such thing as money, sex, etc. and morally obligatory to apportion your highest concern only to worthy objects such as truth and justice. But these views of mine, thought rationally defensible, can be opposed with a show of rationality by sincere people. A hedonist might sincerely and thoughtfully maintain that pleasure alone is the good and that there can be no grounded distinctions among pleasures, and that therefore the pursuit of sexual pleasure among consenting adults is a perfectly legitimate and morally permissible object of pursuit. I would consider it better if people devoted themselves to truth over sexual pleasure, but the abuses that would result from using state power to enforce that view of mine would be worse than the positive outcomes would be good.
For a second example, consider the moral obligation to maintain one's physical health by the usual measures including good diet and vigorous exercise. If you value individual liberty and understand what I wrote above, you will not want laws that mandate these measures. (By the way, this is why 'universal health care,' i.e., government control of the delivery of health services ought to be opposed: a government that controls health care will be in a position to demand 'appropriate' behaviors from people including not engaging in activities deemed risky.)
Ad (P3). If an act is morally permissible, should it also be legally permissible? No. Surely there are circumstances in which running a red light is morally permissible. Suppose at three in the morning you slowly and cautiously coast through a red light at a deserted intersection in the proverbial 'middle of nowhere.' Surely that is morally permissible. Yet it ought not be legally allowed: a well-crafted law cannot have a clause that reads, "A motorist shall not run a red light except when it is safe to do so."
For a second example consider that is it morally permissible for some 15- year-olds to drive. But it ought not be legally permissible for 15-year-olds to drive. The same goes for voting. There are 15-year-olds more mature than some 50-year-olds and more fit to cast a vote. Yet it ought not be legal for 15 -year-olds to vote. Finally, it seems obvious that some marijuana use by some people in some circumstances is morally permissible, but it scarcely follows that marijuana use ought to be legalized. What is harmless for some will be dangerous for many. Laws crafted for the common good cannot take into account individual differences.