St. Thomas Aquinas (1225-1274) points out that “human government,” as St. Paul (Romans 13:1-7) and St. Peter (I Peter 2:11-17) each claims, “is derived from the Divine government.” For this reason, St. Thomas writes, the former should imitate the latter: “Now although God is all-powerful and supremely good, nevertheless He allows certain evils to take place in the universe, which He might prevent, lest, without them, greater goods might be forfeited, or greater evils ensue. Accordingly in human government also, those who are in authority, rightly tolerate certain evils, lest certain goods be lost, or certain greater evils be incurred: thus Augustine says (De Ordine ii, 4): `If you do away with harlots, the world will be convulsed with lust.’”
Although St. Thomas is a pefectionist, in the sense that he believes that a just political regime must be grounded on the principles of the natural law, he sees human law making as an activity of practical reason. It is in that sense St. Thomas is a realist. That is, when crafting human law, legislators must take into consideration the community to which the law is to be applied as well as the universal truth of the fallen nature of human beings. Writes St. Thomas:
Now human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of others, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like….Relying on Scripture and what it teaches about human nature, St. Thomas suggests that modesty and prudence should mark the character and judgments of the legislator. For this reason, the contemporary Christian should resist the temptation to cooperate with those, on either the right or the left, who promise that their policies will actualize a utopian vision of a just society. As we learned all too well from the bloody 20th century, totalizing visions of the state and its powers never end well.
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 (Psalm 30:33): "He that violently bloweth his nose, bringeth out blood"; and (Matthew 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. 
Notes St. Thomas Aquinas, Summa Theologica, II-II, q. 10, art. 11, literally translated by Fathers of the English Dominican Province, 2nd and rev. ed. (1920), online edition (Copyright © 2008 by Kevin Knight), available at http://www.newadvent.org/summa/3010.htm
[2} Ibid, I-II, q. 96, art. 2, available at http://www.newadvent.org/summa/2096.htm