I have offered simple orbital mechanics, and the earth/sun "system", as a metaphor for the importance of the connection between the unitive and procreative aspects of the marital act. I would like to suggest another metaphor to help us understand and appreciate the notion of "moral drift" in the absence of divine replenishment. In analogy with the thermodynamic law of entropy (the second law of thermodynamics), I would like to suggest that there is a kind of moral and spiritual equivalent.
One statement of the law of entropy is that in a closed system (in which energy neither enters nor leaves - the universe being an example) every process that produces work also dissipates heat energy and eventually comes to a halt. In the popular jargon this means that there is no such thing as a perpetual motion machine. Your car needs energy, in the form of gasoline, diesel oil and/or electricity (or in more exotic cases, liquefied natural gas or hydrogen fuel) in order to run the engine. As the engine runs, your car moves and, at the same time, dissipates heat. It is impossible to perfectly recapture the dissipated heat in usable form to produce more useful work. The result is that the energy of the system eventually becomes unusable. (Note: a car with a hybrid gas/electric power system uses a modified braking system to recapture forward motion in the form of electric charge stored in the battery. The efficiency of a hybrid engine is therefore superior to that of a system which, like the conventional internal combustion power system, does not do so. Nevertheless, not all of the kinetic energy of the system can be recaptured and stored for future use when the car is slowed or stopped. Some of that energy is inevitably lost to heat, and most of that heat is inevitably dissipated. The process of recapturing energy is an example of converting kinetic energy to potential energy. Energy is readily converted from kinetic to potential, and vice versa. A heat differential is an example of a potential energy system, since the heat differential helps to provide a differential in pressure. A high entropy system is one in which such differentials are largely absent. Thus when heat is dissipated, that means it spreads throughout the system, dampening any heat differential.)
The earth receives energy from the sun. All the energy of the earth is connected, in some way, with the sun, either in the form of the solar energy released in thermonuclear fusion reactions on and in the sun, or in the much more diminished form of gravitational forces, which produce "tidal friction". The gravitational connection between the earth and the sun suggests (according to Newton's law of gravitation) that the earth will revolve around the sun forever. If the sun did not lose mass (in the form of radiated energy), the earth's orbit would decay due to tidal friction. (It may do so, anyway, for all I know, since I have never encountered a discussion of the balance of forces involved. Certainly, by the time the sun burns out - having exhausted all of its hydrogen fuel in thermonuclear reaction, the earth will either have drifted away or it would eventually fall into the sun. By the time that happens - some billions of years hence - the earth would have cooled to just above absolute zero temperature.) In any case, the earth, and all the life on it, will die. This is thermodynamic inevitability.
The sun provides both light and heat. Both are required for our survival. The heat keeps our bodies warm. It also provides the illumination we need to perform work, and to conduct other activities, during the day. The light provides energy for plants, which convert light, via photosynthesis, into energy.
Just so, the Spirit of God provides light, in the form of illuminated understanding, and heat, in the form of zeal or motivation to do what our understanding informs us is right, for our spiritual and moral survival. A person who hungers and thirsts for righteousness is an example of a high energy moral system, also known as a zealot. A person who is also illuminated in their understanding of what constitutes righteousness is a saint, or on their way to becoming one. (I suppose one could say that a low-motivation, yet illuminated person is a guru who prefers to be a hermit and not be bothered by us mere human beings. I can't think of any examples of saints that would fit that description. Pope Emeritus Benedict XVI wrote the encyclical Caritas in veritate in testimony to the fact that love and truth are natural partners, both required for real flourishing. Truth without love leads to rigidity. Love without truth leads to fanaticism.)
Without divinely illuminated understanding, public morality becomes subject to the whims of popular opinion and/or the mandates of government and cultural elites (which are also subject to "trends"). Without divinely energized love, public practice remains self absorbed, robbed of what we might call (in imitation of tradition) "public spirit". The ideal of a rational economy devoid of any sense of solidarity with the poor, for example, was described by Ayn Rand as Objectivism. We might describe Objectivism as a variant of Nietzscheanism, insofar as the Objectivist philosopher eschews the domination of others. The ideal of a "love" economy devoid of any recognition of market realities (which are, obviously, the result of rational self-interest operating at the micro economic level and aggregated at the macro economic level) is Communism. Communism can be motivated by God or motivated by the super state. Every example in history of Communism attempted on a grand scale has been motivated by a super state governed by elites. Such elites tend toward fanaticism insofar as the avoid cynicism. Neither Objectivism/Nietzscheanism nor Communism has a capacity for building a society where the human person is honored and has an opportunity to flourish. Pope Francis (and a number of other churchmen) have attempted to articulate the former statement (regarding Objectivism/Nietzscheanism) by referring to the failure of "trickle down" economics or "libertarianism". The problem with this latter analysis is that it fails to account for the motivations of the individuals in the system or even to contrast such systems with those principles which might be easily recognized as demonstrably superior, such as the principle of subsidiarity combined with the principle of solidarity. Government theorists need to recognize, of course, that government mandated subsidiarity and solidarity puts the cart before the horse. (We can begin to grasp the truth of this last statement when we consider the history of Constitutional government. Government mandates inevitably drift toward supersidiarity and cronyism, the opposite of the very virtues they would presume to cultivate.)
There is no substitute for divine illumination and inspired love. There is, however, a supernatural culture of incarnated divine inspiration incorporating illumination and love. That culture is the Mystical Body of Christ. The Roman Catholic Church, I believe, provides the spine, and much of the muscle and sinew, skeleton and nervous system, of that Mystical Body. Christ, of course, is the head. To the extent that Churchmen remain close to Christ, they share in his illumination and love. They, and the Godly laity, become the salt of the earth and the light of the world.
Without Christ, moral drift is inevitable. Without the Church, the embodiment of Christ on earth, moral drift is inevitable.
What, you ask, is moral drift? Ultimately, it is a drift toward a condition of moral confusion. It is a drift toward a culture of amorality. Such a culture is characterized by a prevalence of narcissism, a ubiquitous sense of entitlement, hyper individualism, arbitrary police power and/or "political correctness", cronyism, elitism and despotism. Leaders either think of themselves as serving an abstract ideology or as serving themselves. They do not think of themselves, ultimately, as serving the people. As such they are incapable of modeling the Christian ideal of servant leadership. There are apparently no leaders like Cincinnatus or George Washington at the head of government today. (If you want to grasp the meaning of servant leadership in the Judeo-Christian tradition, read the Book of Judges and Philippians 2:1-11, preferably - to get all of the nuance - in the original Greek.) In short, such a system is modeled by the Roman Empire after the ascension of Augustus, with a brief respite in the reign of Constantine, and reaching great depths in the reign of Nero, Caligula, Diocletian and the like (though, notably, each specialized in their own abominations, e.g., Diocletian raised the Roman military and government bureaucracy to new heights).
An amoral culture is a seedbed for abortion, euthanasia, government breeding mandates (i.e., eugenics and population control), ubiquitous governmental interference in daily living (via propaganda [government control of the education system], control of the healthcare system, control of real estate distribution via environmental regulation, death taxes, eminent domain and other legal confiscations and the like), chemical dependency of all forms, prostitution, polygamy, "free love", contraception, IVF, surrogacy, gamete "donation" or sale (not to mention several other sexual/social dysphorias and obsessions), a thriving black market system in slavery, organ donation, vote buying and fraud, despotic/thuggish unions, monopolies, government/business cronyism, etc. In other words, it is precisely where we are headed, today. We have already seen most of these phenomena in large scale. There is little left in the public morality that can reliably hold back the floodgates.
As the Emperor Diocletian, Adolf Hitler and Vladimir Putin discovered, no such culture can survive apart from despotic rule. As history always discovers, no such culture can survive long, regardless of what isolated virtues are raised to prop it up. The only question we seem to have left in our increasingly amoral American culture is who will become that despot?
The record of history, both biblical and secular, makes it abundantly clear that God allows cultures to drift. He is not a despot who forces us to live his way. He offers light and heat to those who "have ears to hear" and "eyes to see" and hearts to feel (the spirit and heart of Ezekiel 36:26), but he does not force them upon us.
Although the record of history may inspire pessimism, yet there is hope. That hope does not, and cannot, lie in anything apart from God.
Today, we live in a period (indeed, we might call it an "age") of moral confusion. It is characterized by division (indeed, growing individualism) and by a general breakdown of public dialog, which, along with a growing tendency to exalt government over individual initiative, tends to stifle any potential for the spread of wisdom. There remain, in such a period, numerous voices crying out in the wilderness. There remain numerous pockets or enclaves of high morality. There remain saints. There will always be saints. Many of them will become martyrs. Look for them. Pay attention to how they live and to what they say (or teach). Look, in other words, for divinely inspired light and heat.
Above all, pray, and do your best to pass on the wisdom of the ages. (I am no Luddite, but passing on wisdom is more important then passing on scientific knowledge.) With God's help, we may shorten the coming Dark Age.