Some comments on the incest thread seem to argue that incest should be banned just because it’s immoral. I don’t think this can be right. There is lots of immoral behavior — cruelty, ingratitude, and a variety of other things — that is not illegal. (On a related point, for a list of violations of the Ten Commandments that aren’t illegal, see here.) It follows, I think, that we have concluded, and rightly so, that mere immorality can’t justify illegality. There has to be something else that supports the judgment.
Before someone is deprived his liberty, we need to have some good reason. Before the taxpayers are required to pay hundreds of thousands of dollars to prosecute and imprison a person, we need to have some good reason. That reason could be direct harm to someone, or indirect harm. But if one can’t identify some such harm, I don’t see why we should lock someone up, or spend lots of your and my money. Naturally, the judgment about what constitutes harm will turn on your understanding of morality; but if you want to use taxpayer money and restrict people’s liberty you should have some more specific reasoning than just “it’s immoral.”
Note that this is separate from the question whether it’s constitutional to ban conduct just because it’s immoral. Lawrence v. Texas held that certain kinds of conduct can’t be banned just on those grounds; but there are plausible arguments against that, and more broadly moral objections may well suffice — even given Lawrence — to justify bans on other kinds of conduct. But just because some ban is constitutional doesn’t mean that it should be enacted.