Human Nature and Aquinas’ Taxonomy of Sexual Sins

St. Thomas Aquinas, in the Second Part of the Second Volume of his Summa theologiae, considers in a little over 1000 pages in Latin a massive number of sins and vices – injustice, gluttony, anger, greed, lying, etc., etc. Sexual sins are considered under the technical scholastic rubric of “luxury” (i.e., lust), and like the other sins are divided up into different species, with numerous concrete examples and applications.

Liberals in our enlightened era generally consider any sexual practice short of rape or child abuse to be “pelvic issues,” not worthy of condemnation, and certainly not able to keep perpetrators from eternal salvation. Aquinas obviously disagrees, but in his taxonomy, while all the sins he considers are “mortal sins,” he ranks them from more to less serious, with reasons for the rankings.  The distinctions he makes are not only important for the insights gained philosophically about the variety of human evils, but also from the standpoint of moral theology.  They were, and remain so today, important for confessors in knowing what types of penances to apportion and for spiritual directors in being able to offer appropriate advice to those who consult them.

It is also important for us to know what such distinctions are.  A lack of distinctions prevails in regard to many sins: Some consider it monstrous for police to beat protesters, but have no problem with radical Islamists massacring women and children indiscriminately. Some are incensed about somebody cheating on food stamps, but not about a politico using insider information to buy stocks. Many are incensed that their favorite movie star’s boyfriend has cheated on her, but have no problem with pornography. Others are against gay sex, but find contraceptive heterosexual intercourse unproblematic. And so on.

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The following listing from Aquinas proceeds from the most serious to the least serious. I will discuss some potentially surprising rankings at the end.

The most serious sexual sins (leaving out circumstances such as violence, which compound the sinfulness) are sins contra naturam, sins contrary to human nature, and thus contrary to God the author of human nature.

The most obviously unnatural sin is “bestiality,” i.e., sexual intercourse with animals – a sin which offers an affront to the human species. Next in seriousness is sodomy, which is an affront to the natural relationship between male and female.

In third place are unnatural coital relationships between men and women – for example, anal intercourse, coitus interruptus, or other contraceptive measures – all of which are sinful because they do not observe “the right manner of copulation.”  In his Summa contra gentiles, Aquinas compares such relationships to homicide: “After the sin of homicide whereby a human nature already in existence is destroyed, this type of sin appears to take next place, for by it the generation of human nature is precluded.” By taking measures to prevent a human life from emerging naturally, such non-procreative sex constitutes an action against the potential human soul that might result.

The least serious “unnatural” sexual sin is masturbation, in which pleasure is intentionally sought in isolation from natural social relationships.  Aquinas is careful to distinguish this from “nocturnal pollution” or other unintentional emission of semen, which is not sinful. In our era, we would include pornography, as a means to excite prurient sexuality, as connected with this sin.

Incest, which is borderline “natural,” if it involves male-female intercourse, is nevertheless a grievous sin since it flouts the natural relationships proper to people connected by consanguinity or affinity.

As regards normal male-female relationships, the most serious sin is of course rape, in which sexual sinfulness is compounded with a serious sin of injustice, forced intercourse with someone who is unwilling.

Next in seriousness is sacrilege, for example, intercourse with a nun or priest who has taken a vow of chastity.  Because of the vow, this sin involves a direct offense to God; and if it is accompanied by rape, the seriousness is compounded.

Less serious is adultery, which is consensual, but is combined with the sin of injustice, since at least one of the parties is joined lawfully to another in marriage.

Finally, Aquinas makes a distinction between two of the least serious sexual sins – “seduction” and “fornication.” In making this distinction he is in part taking into account the customs in his era, in which (as also in our own time) a father at a wedding will “give away” the bride.  The legal code then favored marriage, on condition of parental consent and consent of the bride; in the absence of such consent, civil penalties for seduction were prescribed.

Fornication, i.e. what we call “consensual sex” is defined by Aquinas as intercourse with a woman who is not a virgin, and in which no external aggravating circumstances are relevant – e.g., the use of force, or the use of contraceptives.  This act is sinful because it militates against the social welfare of possible progeny who might result – leading to the possibility of children without a father to aid them with moral and intellectual guidance into adulthood.

We might consider some of these rankings to be counter-intuitive:

Masturbation worse than fornication? The psychiatrist Karl Menninger, in his 1973 book, Whatever became of Sin? in his comments on modern culture, points to the change in attitude regarding masturbation as a pivotal development paving the way to a permissive attitude not only towards sexual sins, but toward sin in general. Without too much imagination we can perceive contraception, sodomy, and pornography as sophisticated cultural results from that change.

Incest more serious than rape? Aquinas’ reasoning is that incest, if it is not accompanied by rape, is still a greater affront to the natural relationship of the sexes, especially when we consider familial relationships between parents and children, or sisters and brothers.

Consensual sodomy worse than incest or rape? Incest disrespects individuals in various degrees of relationship, while sodomy is an infraction against the proper relation of the sexes as well as against the perpetuation of the human species. Intersexual rape in a certain sense is less “unnatural,” but brings in the extraneous factors of violence and injustice which can magnify the overall sinfulness of the action.

Aquinas, of course, was operating in the context of the philosophical supposition that human nature is unchanging, and thus contains certain “constants.” Our sophisticated progressive contemporaries—in particular those in positions of academic and cultural influence—believe quite the opposite. Human nature —if there is such a thing— is infinitely malleable, and sexuality blossoms out in evolutionary fashion into polymorphous re-creations–families without a biological mother and father, liaisons for mutual pleasure without procreation, serial polygamy through successive marriage and divorce, etc.

We can pretend that human nature is not what it is only for so long. An indefinite suppression of reality is not possible. We’d do well to reflect on the Angelic Doctor’s instruction.


  • Howard Kainz

    Howard Kainz is professor emeritus at Marquette University. He is the author of several books, including Natural Law: an Introduction and Reexamination (2004), The Philosophy of Human Nature (2008), and The Existence of God and the Faith-Instinct (2010).

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