Obj. 2: Further, no mortal sin is the matter of a Divine precept. But the Lord commanded (Osee 1:2): "Go take thee a wife of fornications, and have of her children of fornications." Therefore fornication is not a mortal sin.
Obj. 3: Further, no mortal sin is mentioned in Holy Writ without disapprobation. Yet simple fornication is mentioned without disapprobation by Holy Writ in connection with the patriarchs. Thus we read (Gen. 16:4) that Abraham went in to his handmaid Agar; and further on (Gen. 30:5, 9) that Jacob went in to Bala and Zelpha the handmaids of his wives; and again (Gen. 38:18) that Juda was with Thamar whom he thought to be a harlot. Therefore simple fornication is not a mortal sin.
Obj. 4: Further, every mortal sin is contrary to charity. But simple fornication is not contrary to charity, neither as regards the love of God, since it is not a sin directly against God, nor as regards the love of our neighbor, since thereby no one is injured. Therefore simple fornication is not a mortal sin.
Obj. 5: Further, every mortal sin leads to eternal perdition. But simple fornication has not this result: because a gloss of Ambrose [*The quotation is from the Gloss of Peter Lombard, who refers it to St. Ambrose: whereas it is from Hilary the deacon] on 1 Tim. 4:8, "Godliness is profitable to all things," says: "The whole of Christian teaching is summed up in mercy and godliness: if a man conforms to this, even though he gives way to the inconstancy of the flesh, doubtless he will be punished, but he will not perish." Therefore simple fornication is not a mortal sin.
Obj. 6: Further, Augustine says (De Bono Conjug. xvi) that "what food is to the well-being of the body, such is sexual intercourse to the welfare of the human race." But inordinate use of food is not always a mortal sin. Therefore neither is all inordinate sexual intercourse; and this would seem to apply especially to simple fornication, which is the least grievous of the aforesaid species.
On the contrary, It is written (Tob. 4:13): "Take heed to keep thyself . . . from all fornication, and beside thy wife never endure to know a crime." Now crime denotes a mortal sin. Therefore fornication and all intercourse with other than one's wife is a mortal sin.
Further, nothing but mortal sin debars a man from God's kingdom. But fornication debars him, as shown by the words of the Apostle (Gal. 5:21), who after mentioning fornication and certain other vices, adds: "They who do such things shall not obtain the kingdom of God." Therefore simple fornication is a mortal sin.
Further, it is written in the Decretals (XXII, qu. i, can. Praedicandum): "They should know that the same penance is to be enjoined for perjury as for adultery, fornication, and wilful murder and other criminal offenses." Therefore simple fornication is a criminal or mortal sin.
I answer that, Without any doubt we must hold simple fornication to be a mortal sin, notwithstanding that a gloss [*St. Augustine, QQ. in Deut., qu. 37] on Deut. 23:17, says: "This is a prohibition against going with whores, whose vileness is venial." For instead of "venial" it should be "venal," since such is the wanton's trade. In order to make this evident, we must take note that every sin committed directly against human life is a mortal sin. Now simple fornication implies an inordinateness that tends to injure the life of the offspring to be born of this union. For we find in all animals where the upbringing of the offspring needs care of both male and female, that these come together not indeterminately, but the male with a certain female, whether one or several; such is the case with all birds: while, on the other hand, among those animals, where the female alone suffices for the offspring's upbringing, the union is indeterminate, as in the case of dogs and like animals. Now it is evident that the upbringing of a human child requires not only the mother's care for his nourishment, but much more the care of his father as guide and guardian, and under whom he progresses in goods both internal and external. Hence human nature rebels against an indeterminate union of the sexes and demands that a man should be united to a determinate woman and should abide with her a long time or even for a whole lifetime. Hence it is that in the human race the male has a natural solicitude for the certainty of offspring, because on him devolves the upbringing of the child: and this certainly would cease if the union of sexes were indeterminate.
This union with a certain definite woman is called matrimony; which for the above reason is said to belong to the natural law. Since, however, the union of the sexes is directed to the common good of the whole human race, and common goods depend on the law for their determination, as stated above (I-II, Q. 90, A. 2), it follows that this union of man and woman, which is called matrimony, is determined by some law. What this determination is for us will be stated in the Third Part of this work (Suppl., Q. 50, seqq.), where we shall treat of the sacrament of matrimony. Wherefore, since fornication is an indeterminate union of the sexes, as something incompatible with matrimony, it is opposed to the good of the child's upbringing, and consequently it is a mortal sin.