Having concluded that fornication and prostitution were gravel immoral,
.. it would seem obvious that Aquinas would want to engage every force against them, especially civil law. Oddly enough he does not. Instead he notes that the state should allow fornication and prostitution to exist for the sake of the common good. Relying on the well-known passage from Augustine's De ordine, Aquinas advocates tolerance of prostitution by noting: "Accordingly in human government also, those who are in authority rightly tolerate certain evils, lest certain goods be lost, or certain evils be incurred: thus Augustine says [De ordine 2.4]: 'If you do away with harlots, the world will be convulsed with lust.'" If these social practices were to be suppressed, the public reaction might be such as to threaten the peace of society.
Far from being theocrats, as so many vulgar critics of the medievals claim, the medievals like Aquinas were actually in favor of greatly limited civil government, which had little use or purpose beyond the maintenance of peace. Civilized society of course, could not function while wars raged everywhere, so civil governments were tolerated for the maintenance and safety of society. The idea, however, that civil governments should pass regulations governing people's food and medicine, and then have such regulations enforced with an army of bureaucrats, would have seemed ridiculous to the medieval mind.
.. in the not-too-distant future, we Catholics will once again be left on our own, as we were for most of the past 2,000 years, and this short age of lazily invoking the state to fight our battles for us will be over.