A good question and discussion over at WorldMagBlog: “Should everything that’s immoral be illegal, regulated, or punished? If so, by which kind of government (include family and church as kinds of governments)? Can you give an example of a behavior that’s immoral but shouldn’t be regulated by the state?”
Here’s what Aquinas has to say on this (in part), and I think it has a lot of merit in determining when and in what situations conduct should be legally sanctioned:
“The purpose of human law is to lead men to virtue, not suddenly, but gradually. Wherefore it does not lay upon the multitude of imperfect men the burdens of those who are already virtuous, viz. that they should abstain from all evil. Otherwise these imperfect ones, being unable to bear such precepts, would break out into yet greater evils: thus it is written (Pr. 30:33): ‘He that violently bloweth his nose, bringeth out blood'; and (Mt. 9:17) that if ‘new wine,’ i.e. precepts of a perfect life, ‘is put into old bottles,’ i.e. into imperfect men, ‘the bottles break, and the wine runneth out,’ i.e. the precepts are despised, and those men, from contempt, break into evils worse still” (Summa Theologica, II.1.96.ii).
The point is that in cases where the law would cause greater evil to be done, it is not prudent to criminalize the behavior. Prohibition strikes me as a particularly excellent example of this.