As a young priest in the late 1960s and early ’70s, I saw firsthand the moral anguish of married couples wrestling with this teaching. I believe their acute pain was intimately tied to their fear of committing mortal sin. We might have had a very different moral discussion following the birth-control encyclical if the church had not insisted that all forms of artificial birth control were intrinsically evil and therefore mortal sins. Labeling human moral acts and omissions that miss the mark as mortal sin always ups the ante—and threatens the credibility of the church’s teaching authority itself.
The essential distinction between mortal and venial sins, noted by (e.g.) Aquinas, is that wrongdoing is mortal if it is wrong because it is inconsistent with love of God and neighbor, and venial if it is wrong because it guarantees a falling short in love of God and neighbor. Thus the distinction is not a matter of how things are labeled, nor is it applied as a 'motivator'. One of the way Cozzens muddies the waters is by chatting about sins of disobedience -- not eating meat on Fridays, a priest not praying the Divine Office, attending Mass on Sundays -- whose status as mortal sins may change over time. But these change because they are actions that are not in themselves mortal sins at all; it's defiantly refusing to do them when they are required that is the problem. (It's likewise the case, with anything that is a mortal sin considered simply on its own, that circumstances may combine in such a way that it is only venial in a particular case, due to ignorance, confusion, pressure, or any number of other things.)
Since the heart of any genuine Christian morality is charity, the Church has a moral obligation to teach people about the ways in which their actions can be inconsistent with, or impediments to, true charity. This does not magically go away, ever; it does not go away if people stop believing it, it does not go away if people do not listen to it, it does not go away if the Church also explores other ways of approaching ethical matters.
What really gets me about Cozzens's article, though, is this:
Catholic moral tradition, especially in the arena of sexuality, remains married to a calculus of sin. Confessors, at least from the time of the Council of Trent, were trained to distinguish between venial matter and grave matter in hearing confessions. That led in turn to an emphasis on the “act committed” rather than on the penitent’s encounter with the healing mercy of Jesus Christ and his or her overall moral orientation. Pope Francis, in harmony with the work of contemporary theologians like Bernard Häring, Charles Curran, Margaret Farley and others, is showing us how to move beyond the narrow legalisms of act-centered morality.
But it seems that many Catholics have already managed to climb out of the dark hole of an act-centered, sin-focused morality all by themselves. They have not lost a healthy sense of sin, but they don’t think a second glance at their neighbor’s spouse or missing Mass on Sunday separates them from God’s grace. Nor do they believe that doing what is necessary to determine the size of their family is always mortally sinful.
The divorcing of actions from "overall moral orienation" -- an "overall moral orientation" to what, if not practical actions, known by what, if not by actions? -- and of actions from mercy toward people -- the "healing mercy of Jesus Christ" for what, if not the damage we cause ourselves and others by actions? -- is bizarre. The test of "moral orientation" is what we actually do; you can think yourself the most splendid person, and it means nothing unless your actions actually show it; you can have the most moral attitude ever, and it is hypocrisy if you do not make an effort to act appropriately to it. There is a reason why ethics is focused on acts; and there are reasons why theological ethics, in particular, tends to focus on acts. None of these are addressed in Cozzens's fantasy, which manages not actually to touch on any ethical points of substance, whether philosophical or theological. The whole thing is carried out in a void in which all actual ethical work outside a very narrow stream obsessed with avoiding legalisms is ignored.