TWO adults enter a room, agree a price, and have sex. Has either committed a crime? Common sense suggests not: sex is not illegal in itself, and the fact that money has changed hands does not turn a private act into a social menace. If both parties consent, it is hard to see how either is a victim. But prostitution has rarely been treated as just another transaction, or even as a run-of-the-mill crime: the oldest profession is also the oldest pretext for outraged moralising and unrealistic lawmaking devised by man. In recent years, governments have tended to bother with...