In general we consider something to be legitimate when it is operating in accord with the laws and rules which properly govern that thing. In this way, we say that a person acts legitimately when he operates in accord with the laws of the state in which he dwells. In this is perhaps the first meaning of the word. However, states likewise have a legitimacy or illegitimacy by which they must be measured. Since Locke, many have seen that legitimacy as something “derived from the consent of the governed.” Some, such as Max Weber with his notion of “Legitimitätsglaube”, exclude all normative criteria for legitimacy and reduce it to a set of beliefs that a people have about their government. Such notions, however, are (finally) excluded when one turns to the teachings of the Church summarized well by Pope Saint John XXIII, “Since God made men social by nature, and since no society can hold together unless some one be over all, directing all to strive earnestly for the common good, every civilized community must have a ruling authority, and this authority, no less than society itself, has its source in nature, and has, consequently, God for its author.” Political legitimacy, therefore, must be derived from God.
Now, the Legitimacy of governments is necessary to determine if one is to act in due obedience to said government. Legitimacy of a government must be determined by a rule or law that is the measure of that government, and cannot be determined by something which is measured or ruled by the government. Further, the legitimacy of something must be judged according to its proper operations; but the proper operation of a government is care for the common good through legislation, and enforcement of legislation, etc. Therefore, if a government enacts legislation or enforces legislation in a manner that is violation of the rule or law by which said government is duly measured, that government, insofar as it acts thus, acts illegitimately. Therefore, the proper measure and rule for governments of nations/states is the eternal and divine law as manifested in natural law. Since the manifestation of what is according to divine law (outside of Revelation) is made present in the natural law. Therefore, to the extent that a government violates natural law in its legislation and decrees, it makes itself illegitimate.
Currently, in the United States of America, the question of “gay marriage” has come before the Supreme Court in such a way that the decision of the court will likely determine in a somewhat definitive and permanent way, whether gay marriage becomes part of “the law of the land.” Such a decision is of grave importance to the legitimacy of the government. Marriage, while duly and reasonably encouraged and in someway regulated by the state, nevertheless is not subject to political deliberation. Marriage is a “pre-political” institution.
By pre-political, what I mean is that marriage must be assumed by just and legitimate governments as a principle by which just and healthy societies must depend. In this way, it is much like the principle of the inherent dignity of human life. A government cannot legitimately authorize murder. To the extent that it does, it shows itself to be illegitimate. So also, a government cannot legitimately authorize marriage that is not between one man and one woman. To the extent that it does, it shows itself to be illegitimate.
To the extent that a government is illegitimate, its decrees and legislation have no authority. The reason for this is that all authority is derived from God. Authority, therefore, is only possessed when one adheres to the eternal law as expressed by the natural law. Violation of the natural law, therefore, shows that one has forsaken one’s source of authority and that, consequently, one has no authority at all. What this means is that by declaring two men, or two women to be married, the U.S. government would manifestly be acting against natural law such that it has no authority to recognize marriage at all. All its decrees and legislation regarding marriage are justly and necessarily to be held in contempt. Marriage as a legally recognized institution will be utterly meaningless in the United States.
The frightening and sobering considerations that follow hard on the heels of the above argument concern when a government becomes wholly illegitimate such that its citizens become duty-bound to establish a new government. Already we see that the United States government has shown itself illegitimate in questions of religion by rejecting the due influence worship of God ought to have in society; it is illegitimate in questions of life through its sanctioning of abortion; and now, apparently, soon it will show itself illegitimate with regard to marriage. How many pre-political societal principles must be violated before the government is rendered wholly illegitimate?
There is some, hope, however. In the past, the United States government showed itself to be in some measure illegitimate with its sanctioning of a race-based institution of slavery and then segregation. Through much effort, these gross violations of natural law were overcome. It seems right, therefore, that all citizens of the United States should see themselves duty-bound, to strive with all their might to bring their government back to legitimacy. There can be no peace under an illegitimate government. What is more, there should be no peace under an illegitimate government. These sobering and frightening reflections should instill fear in the hearts of our leaders and all citizens. When a government is no longer acting legitimately and it forfeits its authority, then this is precisely when mob-rule or tyrants take control.
 Cf. Max Weber, The Theory of Social and Economic Organization, ed. Talcott Parsons ed. (New York: Free Press, 1964), 382.
 John XXIII, Encyclical Letter Pacem in Terris: AAS 55 (1963), 269; Leo XIII, Encyclical Letter Immortale Dei, in Acta Leonis XIII, V, 1885, 120.
 Cf. Aquinas, Summa Theologiae, I-IIae, Q. 94, Q. 95, and Q. 96, a5.
 There are also a-political questions that are similarly not properly subject to political deliberation. An example of such a question that nevertheless has been a frequent object of political deliberation in the United States is the theory of evolution. While it is conceivable that such questions could legitimately be discussed in a political context if they pertain to the common good, nevertheless, this would be a sign of a societal malady, and should be avoided as much as possible.
 Cf. Rom. 13:1, and Mt. 28:18
 My focus here is on domestic matters, but one could likewise include unjust wars and other illegitimate activity in foreign countries in this list, but this would involve larger discussions about sovereignty etc.