Glad to broach every topic, human and divine, the scholastic Thomas Aquinas (1225–1274) took up the issue of sex in his Summa against the Gentiles. He wrote this work around 1260 to provide arguments against the scientific views of—among others—elite Muslim scholars of ancient Greek learning, such as Averroes. The section on sex came when Thomas took up issues involved in living a moral life. As usual, he first offered arguments [here 1–3] for the position that he disagreed with: that sex outside of marriage (“fornication”) was not a sin. Then he offered a long rebuttal [excerpted here as 4–6].

The Reason Why Simple Fornication Is a Sin According to Divine Law, and That Matrimony Is Natural

[1] . . . We can see the futility of the argument of certain people who say that simple fornication is not a sin. For they say: Suppose there is a woman who is not married, or under the control of any man, either her father or another man. Now, if a man performs the sexual act with her, and she is willing, he does not injure her, because she favors the action and she has control over her own body. Nor does he injure any other person, because she is understood to be under no other person’s control. So, this does not seem to be a sin.

[2] Now, to say that he injures God would not seem to be an adequate answer [against this argument]. For we do not offend God except by doing something contrary to our own good, as has been said. But this does not appear contrary to man’s good. Hence, on this basis, no injury seems to be done to God.

[3] Likewise, it also would seem an inadequate answer to say that some injury is done to one’s neighbor by this action, inasmuch as he may be scandalized. Indeed, it is possible for him to be scandalized by something which is not in itself a sin. In this event, the act would be accidentally sinful. But our problem is not whether simple fornication is accidentally a sin, but whether it is so essentially.

[4] Hence, we must look for a solution in our earlier considerations. We have said that God exercises care over every person on the basis of what is good for him. Now, it is good for each person to attain his end, whereas it is bad for him to swerve away from his proper end. Now, this should be considered applicable to the parts, just as it is to the whole being; for instance, each and every part of man, and every one of his acts, should attain the proper end. Now, though the male semen is superfluous in regard to the preservation of the individual, it is nevertheless necessary in regard to the propagation of the species. Other superfluous things, such as excrement, urine, sweat, and such things, are not at all necessary; hence, their emission contributes to man’s good. Now, this is not what is sought in the case of semen, but, rather, to emit it for the purpose of generation, to which purpose the sexual act is directed. But man’s generative process would be frustrated unless it were followed by proper nutrition, because the offspring would not survive if proper nutrition were withheld. Therefore, the emission of semen ought to be so ordered that it will result in both the production of the proper offspring and in the upbringing of this offspring.

[5] It is evident from this that every emission of semen, in such a way that generation cannot follow, is contrary to the good for man. And if this be done deliberately, it must be a sin. Now, I am speaking of a way from which, in itself, generation could not result: such would be any emission of semen apart from the natural union of male and female. For which reason, sins of this type are called contrary to nature. But, if by accident generation cannot result from the emission of semen, then this is not a reason for it being against nature, or a sin; as for instance, if the woman happens to be sterile.

[6] Likewise, it must also be contrary to the good for man if the semen be emitted under conditions such that generation could result but the proper upbringing would be prevented. We should take into consideration the fact that, among some animals where the female is able to take care of the upbringing of offspring, male and female do not remain together for any time after the act of generation. This is obviously the case with dogs. But in the case of animals of which the female is not able to provide for the upbringing of offspring, the male and female do stay together after the act of generation as long as is necessary for the upbringing and instruction of the offspring. Examples are found among certain species of birds whose young are not able to seek out food for themselves immediately after hatching. In fact, since a bird does not nourish its young with milk, made available by nature as it were, as occurs in the case of quadrupeds, but the bird must look elsewhere for food for its young, and since besides this it must protect them by sitting on them, the female is not able to do this by herself. So, as a result of divine providence, there is naturally implanted in the male of these animals a tendency to remain with the female in order to bring up the young. Now, it is abundantly evident that the female in the human species is not at all able to take care of the upbringing of offspring by herself, since the needs of human life demand many things which cannot be provided by one person alone. Therefore, it is appropriate to human nature that a man remain together with a woman after the generative act, and not leave her immediately to have such relations with another woman, as is the practice with fornicators.

Source: Thomas Aquinas, Summa contra Gentiles, book 3, Part II. Translated by Vernon J. Bourke at http://dhspriory.org/thomas/ContraGentiles3b.htm#122.

Question to Consider

Why might Thomas have based his arguments against fornication on what is good for man rather than on citations from the Bible?