I would be willing to bet that when you hear the term “mortal sin,” one of the first examples that comes to mind likely involves something sexual. This instinct is not without merit. After all, the Church teaches that sexual sins committed outside of marriage are, by their nature, always grave matter and therefore are always mortal to the soul when committed with full knowledge and deliberate consent. There is a reason why, for example, the word “immorality” in Scripture is usually a euphemism for sexual sin. But this clarity about sexual sin outside of marriage can sometimes lead to confusion about sexual sin within marriage. This leads many to ask, are all sexual sins between spouses also mortal sins? To answer that question, we need to focus on what differentiates the sinfulness of sexual sins outside of marriage from sexual sins within marriage.
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When we are talking about sexual sins outside of marriage, there is a direct offense against the dignity of marriage as a whole when these sins are committed. For that reason, they cannot admit of any slight matter whatsoever. We must remember that God instituted the Sacrament of Marriage to be an archetypal sign not only of His covenantal union with us but even of the inner life of the Trinity itself. Therefore, to attack the dignity of marriage by engaging in sexual activity outside of this sacramental covenant is to distort the sacred Icon that God intended marriage to signify. Such act (e.g., fornication, adultery, pornography, or homosexual acts, etc.) do violace against the very essence of what marriage is: a total, faithful, fruitful, and freely given union between one man and one woman. Because these acts fundamentally contradict the dignity of this Sacrament, they are always objectively grave.
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But within marriage, the moral evaluation of sexual acts takes on an entirely new center of gravity. Morality in marital intimacy depends not on the existence of sexual activity itself, but on how that divinely sanctioned activity is actually ordered in practice. The gravity of sexual sins within marriage therefore is going to depend on what truly honors or dishonors the ends of marriage, namely, the unitive and procreative purposes established by God.
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Within this framework, then, we can pretty easily discern what kind of violations of marital chastity would be mortally sinful: contraception, because it directly opposes the end of procreation by rendering the sexual act sterile; masturbation, because it directly opposes both the ends of procreation and unitive love; sterilization, because it renders the procreative end permanently unfruitful, etc. These sins are mortal not because they are sexual, but because they constitute a deliberate rejection of the very purposes for which God designed marriage and sexuality in the first place. In such cases, the marital act is not merely distorted—it becomes a false sign, no longer reflecting the fecund love of the Creator.
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But is this true for all sexual sins within marriage? St. Augustine, St. Thomas Aquinas, and with them the long tradition of moral theology, would argue no. There are indeed other kinds of sexual sins, primarily having to do with disorders of affection, selfishness, or immoderation, which, while not ideal, simply do not rise to the level of mortal sin. The reason for this is that there is a fundamental difference between opposing a good and enjoying a good inordinately or with an imperfect ordering of motivations.
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For instance, sometimes, the unitive and procreative ends of marriage are respected or at least not impeded, but there is still an inordinate preoccupation with pleasure that is valued more than fostering love for the other spouse as a person. In these cases, the disorder lies not in the direct ordering of the act itself—since it is still open to life and involves true union—but in the source of motivation or relative lack of self-mastery with which the act is carried out. As St. Thomas notes, when spouses engage in the marital act primarily for physical gratification, they commit a venial sin, but not a mortal one, because the act remains fundamentally ordered toward the goods of marriage.
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In fact, St. Thomas goes so far as to say that these venial sins within marriage may be even less serious than gluttony1, since gluttony actually indulges the lower passions more directly without any real ordering, imperfect or otherwise, toward the common good of society, which is not the case with marriage.
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That being said, we should always take care to root out venial sin in our lives. We are called not merely to avoid mortal sin, but to strive for the heights of sanctity. The purpose of making these distinctions is not to provide any license to sin, even venial sin. Rather, it is to offer moral clarity to those who may be tempted to scrupulosity or even despair, thinking that any inordinate enjoyment of the marital act, any imperfection in motivation, or even any irregularity in habit in expressing sexual love within marriage could cause a loss of salvation.
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But this kind of extreme rigorism is not what Christ desires for us. As He Himself says, “My yoke is easy and my burden is light” (Mt. 11:30). The commandments of God are demanding, but they are not impossible. And the grace of the sacrament of marriage is given not only to sustain us in love, but to heal us when we fall short. In short, marital chastity is not a matter of suppressing eros, but of sanctifying it. In this way, even the imperfections along the journey can become opportunities for gradual growth in ever greater holiness.
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I conclude with the advice given by the great Fr. John Hardon, who said:
“Husband and wife must learn that their chastity is very different from what it was before they married. The whole state of their life is changed. They are neither brother nor sister, neither master nor servant. Before God, they will be held responsible for many things that do not concern the unmarried; their duties are different. For this reason, He has attached numerous privileges to the married state that no one else may legitimately enjoy.”2
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1. Thomas Aquinas, De Malo, q. 15, a. 2, ad 16.
2. Fr. John A. Hardon, SJ, Moral Theology, Chapter VII: "Birth Control"
