I think it depends on the significance of the action. Some actions we regard as immoral but not terribly important. The person sitting next to me in the library on the computer is annoying me giggling at Facebook with her companion, and I suppose this is immoral, since she has a duty not to disturb others in a public space in this way, but hardly something that should be illegal, although the idea of her being hauled away by the police is not an unpleasant one to me.

For liberals it is the notion of harm that is significant. If you think that abortion causes harm then it would be difficult to argue that the law should not be involved. Of course, marital infidelity also causes harm and there are countries where it is harshly punishable. Another highly relevant issue is whether one thinks that punishing behaviour will be effective in controlling it. So one may think an action is immoral and should be punished by the state, but it is not likely to be effective to try to do so. Which means there is no point in making it illegal.

There's the old saying that ignorance of the law is no excuse, because it's an excuse that anyone could offer and we wouldnm't know how to refute them. Legally, things are a bit more complicated. I gather that the Due Process clause of the US Constitution carves out some exceptions. If there's nothing "obviously" illegal about a certain kind of conduct, and the State doesn't provide proper notice to citizens that it's against the law, then the law won't pass constitutional muster. A fanciful example: suppose that buried in the bowels of some omnibus bill was a provision making it illegal to drive a red-and-blue car, but the State made no effort to let people know. Fining someone for this new "offence" would probably not stand up to challenge.

So in US law, at least, there's some requirement that citizens have a reasonable chance of knowing what's illegal. But even supposing all the laws were properly promulgated, it's not clear that we have an actual duty to know and understand them all.

Suppose I want to set up a complicated business. I don't know all the relevant laws, and I certainly don't understand them all. But I do know that this is tricky territory. I can be expected to know that there might be various licensing requirements, certain tax provision that I'd be subject to and so on. If I go ahead without consulting a lawyer and break these laws left and right, I won't be able to plead ignorance. But what the law is really expecting of me is not that I become an expert on all this. It expects me to take reasonable means to make sure I'm in compliance with the law. And there's an obvious way to do that short of gaining the knowledge myself: talk to an attorney. I can take reasonable steps to ensure that my business is legal even though I personally have only a faint understanding of what the law entails. And in typical cases, at least, that's what keeps the dilemma at bay.

More than on your preferences, the answers also depend on the kind of students you face and on the legal system within which they serve. In light of my limited knowledge of these and other relevant matters, I would suggest you focus on leading your students to think philosophically about the law. For example, what moral authority do those in government have to enforce laws against non-consenters? What must the government be like, and what must the laws it is enforcing be like, for such enforcement to be morally permissible? And under what conditions does the mere fact that something is the law give citizens a moral (as opposed to a merely prudential) reason to act accordingly?

These sort of questions and reflections are crucial, I think, for students to appreciate the conceptual gap between the law and justice -- a gap that is often deliberately obscured, as when the government agency in charge of law enforcement is called the Department of Justice (its recent head in the US, Attorney General Alberto Gonzales, authorized torture) and judges are referred to as justices. Being aware of this gap helps future lawyers to be sensitive to the responsibilities they bear, as officers of the law, for its broad conformity to justice.

Such broad conformity is endangered when lawyers and other officials act "under color of law," i.e. abuse their legal authority for personal ends. And such broad conformity is even more seriously endangered when the entire legal system is perverted in the service of a blatantly unjust regime, as arguably happened during Brazil's two decades of military dictatorship. As St. Augustin famously said (in somewhat different words), without justice a well-ordered legal system may be no better than organized robbery and exploitation. Such perversion is, of course, a matter of degree. And so the lawyer's responsibility always involves helping to make the legal system one that really has moral authority to command and really does generate moral obligations to comply. In Brazil, as in most other countries, a great deal remains to be done, and lawyers can play an important role in promoting not merely the rule of law, but the rule of just law.

I suppose there is generally a distinction between actually doing something and doing it in order to represent it "artistically". Suppose for example that I set out to cheat passersby by operating a scam; then if I am caught I may be prosecuted and sent to prison. If I act in a movie in which I do exactly the same thing, I would not be charged since although I am being paid to represent something in itself illegal, the representation is not itself illegal.

None of this of course suggests that it is a good idea to have laws against prostitition.

Is it true that all states in which prostutition is illegal also legally permit the making of hard-core pornography in which performers are paid to engage in sex with each other? Surely there are some states that prohibit prostitution but do not ban (or at least do not prosecute) the making of pornography (California). But there also might well be states that prohibit both prostitution and the making of pornography, and prohibit the latter using the laws against the former. We need to do some legal research. I know that one feminist legal argument that tried to bring legal pressure to bear on pornography, without going the controversial route of the MacKinnon-Dworkin Ordinance back in the 1980s and 1990s, emphasized that the making of much pornography involved prostitution and hence could already be prosecuted under existing state laws. I do not know whether any jurisdictions capitalized on this argument in fighting pornography (either from a feminist or socially conservative perspective). Another complication is that in many jurisdictions prostitution is merely a misdemeanor and laws against it are rarely or infrequently enforced. That lack of zeal might explain why pornography can be made without legal worries. The making of pornography is also less visible---carried out indoors, and in private places---than streetwalking-style prostitution, and may evade prosecution that way, too. The philosophical (analytic and moral) questions (versus the legal ones) about the relationship between prostitution and pornography are also interesting. I took a stab at it 20 years ago in my Pornography (Yale University Press), especially pp. 127-35.

The two statements in quotes are surely different. But the first can also express a moral standpoint: that it is morally important to achieve a low murder rate. This moral standpoint is reflected in various more specific claims.

1. We should not inflict punishment or pain on anyone unless doing so produces some good for others (e.g., by preventing the person from offending again or by deterring others).

2. We should inflict pain whenever doing so produces some greater good for others.

This second claim is highly problematic insofar as it may justify "punishing" the innocent when doing so helps deter real criminals. For this reason, those who hold deterrence to be morally important often claim instead:

3. In deciding how severely to punish specific types of crime, we should take into account how much of an impact greater severity would have on the frequency of this crime.

This third claim is consistent with the idea that people may be punished only for having done something wrong. But it allows two kinds of conduct that are equally wrong to be punished differently. One crime is punished lightly, yet some equally bad crime is punished more severely because here such punishment is more effective in getting the crime rate down. Moral arguments are made both for and against this claim.

By way of a partial answer: the phrase 'against his will' in the quotation places the moral stress on the notion of 'informed consent'. But such consent is not an uncontroversial concept. It may that 'the state' has a duty of care with respect to those whose -- or in those situations where -- consent is not or could not be 'informed'. Please see my answer to a previous question for a bit more on this:

http://www.amherst.edu/askphilosophers/question/1157

You raise a good point about the rather arbitrary fixing of age limits for various activities. Of course there will always be those over the limit who do not really understand what they are doing, and those under who do, but that is inevitable in any rough and ready measure based on age. The answer is often that it is better to have an inaccurate cut off point than no cut off point at all, since we might reasonably expect that most under-sixteens, say, would not really know what was involved in getting married, even though some exceptionally mature sixteen year olds might. It is better to have some sort of rule like this than to have no rule at all, since if people were able to marry at any age the scope for exploitation would be increased. Similarly with voting, there are plenty of idiots of mature age who have the vote, while thoughtful and intelligent young people do not, but that seems fair, since unless everyone is going to be allowed to vote some restrictions are going to have to apply. And it is better to have them age-based than based on anything else. Some strange anomalies result, of course, such as that in many countries young people are able to join the armed services and kill people but not allowed to drink alcohol!

I don't know about informed consent, though, since you seem to think that you have to have experience of an activity before you can give informed consent. Suppose someone has never smoked, and refuses a cigarette, it seems strange to say that they don't really know what they are refusing. They don't know what a cigarette actually tastes like, but that is all. I don't know what pork tastes like, since I have never eaten it, but I think when I refuse it I am giving my informed consent since I know the sort of thing it is. It is difficult to argue that a young person in Western culture does not know what sort of activity sex is, even without participating in it directly, since it is all around us in one form or another. An individual minor could morally speaking be in a position to give informed consent to sex, but the law has to deal in generalities, and it is to the advantage of society as a whole that certain age ranges are used to differentiate the legality of sexual behavior. Proving that someone could really give informed consent is a difficult and lengthy business. Proving they are a certain age is quite quick and simple.