Augustine and Aquinas opposed legislation against prostitution because such a prohibition would produce great civil discord, not merely a moderate amount of disagreement.  Aquinas followed Augustine in saying that "If you do away with harlots, the world will be convulsed with lust."  Public morality has probably changed since then.  The world would not be "convulsed with lust" if prostitution were prohibited; nowadays, sex is free.  Thus, Augustine's and Aquinas's rationale for prohibiting prostitution disappears.  Almost.  They were also against prohibiting acts that people are hell-bent on committing.  Such prohibitions are futile, and in the end, breed disrespect for the law.   This is not good for the state.