Which is worse: adultery or masturbation? Rape or masturbation? Did Aquinas teach that masturbation is a greater sin than rape, because masturbation is unnatural, and rape is not? If so, what did he mean by speaking in this? The suggestion that masturbation is ultimately a greater sin than adultery or rape is immediately repellent to us. In fact it is ridiculous, yet certain authors attribute precisely this position to Thomas Aquinas, reading him to be saying that the sin of masturbation is simply speaking, ultimately, worse than the sins of adultery and rape. Moreover, the further argument is made that the Church upheld this position for a long time, and this (false claim) is used to attack the Church's credibility in sexual ethics. Doubt is sometimes expressed regarding various teachings or practices of the Church, e.g., regarding the Church's affirmation that homosexual intercourse is wrong, or the restriction of priestly ordination in the Roman Rite to those who freely choose to embrace celibacy. The argument is made that if the Church "taught for a thousand years that masturbation is a worse sin than rape", its teaching on sexual matters can't be very sound.

This misunderstanding of the Church's traditional teaching on sins against nature seems to be more prevalent than I realized. I hadn't previously realized how many authors and teachers assert that not only many medieval theologians, but even Thomas Aquinas taught this. Here are some sample quotes from books discussing masturbation in Aquinas:

[In Aquinas's view], Because sins against nature were sins against God, they were considered more serious than sins against other people, such as adultery, seduction, and rape (John F. Schumaker, Religion and Mental Health [Oxford University Press US], 1992), 76).To make his point perfectly clear, Aquinas poses a question: are not rape and adultery worse than unnatural acts, since they harm other persons, while consensual sins against nature do not? The answer is unequivocal: the four non-procreative forms of sex are worse, since–though not harmful to others–they are sins directly against God himself as the creator of nature. According to this logic, rape, which may at least lead to pregnancy, becomes a less serious sin than masturbation (Louis Crompton, Homosexuality and Civilisation, [Harvard University Press, 2006], 188). "A practice opposed to the pattern set for us by nature" exceeds in wickedness the seduction of an innocent of the opposite sex, adultery, and rape (II-II 154:12) (Sex from Plato to Paglia, by Alan Soble [Greenwood Publishing Group, 2006], 1053).

One unfamiliar with the scholastic manner of speaking formally about an issue, that is, addressing precisely the question at hand, might easily get this impression from Aquinas's treatment of sexual "sins against nature."

Here is the text itself. In the Summa Theologiae II-II, q. 154, a. 12, Aquinas says:

In each kind of thing the worst corruption is the corruption of the principle on which other things depend. Now the principles of reason are the things in accord in nature… and therefore, to act against what is determined by nature, is most serious and base. Therefore since in the sins against nature man transgress what is determined by nature in regard to sex, the sin in this matter is the gravest kind of sin. After this is incest… while by the other species of lust one transgresses only that which is determined according to right reason, but presupposing the natural principles. But it is more contrary to reason to have sex not only contrary to the good of the offspring to be born, but also with injury to another. And therefore simple fornication, which is committed without injury to another person, is the least kind of lust.

The first objection of the article argues that sins against nature are not the worst, because they are not the most contrary to charity: "The more a sin is contrary to charity the graver it is. Now adultery, seduction and rape, which are injurious to our neighbor, seem to be more contrary to the love of our neighbor, than unnatural sins, by which no other person is injured. Therefore sin against nature is not the greatest among the species of lust." St. Thomas replies to this objection: "As the order of right reason is from man, so the order of nature is from God himself. And therefore in sins against nature, in which the very order of nature is violated, injury is done to God himself, the one who ordains nature."

Aquinas is focusing on the sins precisely as a violation of the right use of sexuality, and abstracting from other aspects of them. As justice is a greater virtue than chastity, so injustice is a greater evil than unchastity, and thus all things considered, Aquinas would consider rape a greater evil than masturbation or contraception. This formal way of speaking is recognized by some more considerate authors:

The teaching of medieval theologians that such sexual sins as masturbation, sodomy, and contraception are more perverse, as sexual sins, than fornication or adultery or even rape (the former were said to be contra naturam whereas the latter were said to be praeter naturam), angers many people today. But this teaching must be understood properly. The medieval theologians are claiming that certain kinds of sexual sins more seriously offend the virtue of chastity than do others. They are not saying that these sins are for this reason less grave as sins than adultery or rape, for instance. After all, adultery and rape are very serious violations of the virtue of justice as well as being violations of the virtue of chastity. Thus, as a sin, rape is far more serious than masturbation or homosexual sodomy because it not only offends chastity but also gravely violates justice. (Ronald David Lawler, Joseph M. Boyle, William E. May, Catholic sexual ethics: a summary, explanation & defense).

It is important to understand this formal way of speaking, considering one aspect of human behavior and abstracting from other aspects. But as Aquinas himself says in another context, "where such a manner of speaking is found in the writings of an authority, one should not continue to speak in this manner, but should piously explain what is said" (Summa Theologiae I, q. 31, a. 4, loose translation).

In regards to the comparison between fornication and masturbation, St. Thomas may have in mind fornication in an instance where there is a potential marriage between the two persons (even if they are not engaged), since otherwise it would be contrary to the good of the offspring who might be born. Whether he had this in mind or not, it does seem to me that there are many cases where fornication is worse than masturbation, on account of the harm done to the other party, and on account of the potential harm of children conceived and born as a result (e.g., if they are thereby deprived of the presence of one parent in the home).

The Reception of Thomas Aquinas

I would agree that Thomas Aquinas's classification of sins as received by moralists in following centuries became a problem, in part because it was not used formally, but materially; that is, the descriptions of sins and the various gravity of different sins or aspects of sins were applied directly to physical, material acts. Even Alphonsus Liguori, named the patron of confessors and moralists by Pope Pius XII, seems to have fallen into this error. And in this respect I do see problems with the Church's usual approach to sexual morality for centuries, not sexual morality in particular however, but simply morality in general, the tendency being to take an extrinsic, legalistic approach to morality, and to focus on sins, rather than an approach focused on the real nature of human goods. This was to some extent an outgrowth of nominalism, which made of natural law a kind of arbitrary imposition by God's will. In another post I'll come back to this general topic of legalistic morality.

Update:

I gather from the questions that bring people to this post that this anachronistic reading of Thomas Aquinas (and the other scholastics) is a common one. Aside from some general queries such as "contra naturam in Aquinas," or "Aquinas' thoughts on rape," or "Aquinas on sexual morality," "Thomas Aquinas on masturbation," "Aquinas' views on rape," there are a number of questions that suggest having encountered such a view:

1. When did rape become worse than masturbation? (Related questions: Rape is not as bad as masturbation? Is masturbation a greater sin than rape? Masturbation worse than incest? Which is worse, masturbation or adultery?)

It was all along. But if the question is meant in the sense "when did people stop making technical lists of how fundamentally an act goes against the sexual purpose of the genital organs?", the answer is, "when they in general stopped making such technical lists."

2. Is masturbation worse than fornication? (Similar related queries: Is masturbation a greater sin than having sex? [This must mean "having sex" outside of a marital relationship.]; is masturbation better or worse than actual intercourse?)

In most cases not. First, as noted above, in the many cases where the present relationship or the capacity for a committed and unselfish relationship in the future is seriously harmed, or where danger to offspring is present, fornication is objectively graver. Moreover, masturbation requires, and in most cases involves, a much less deliberate act of will than fornication does, and therefore the sin is any case subjectively less bad.

3. The catholic church taught that masturbation is worse than rape because at least the latter might result in conception. (Related questions: Rape is much less a sin than masturbation. Masturbation is worse than rape? [Did] Aquinas [teach that] rape is better than masturbation?)

No, it didn't.

4. Is masturbation worse than adultery?

No.