Further, as stated above (Q[41], A[1]), marriage is natural. But this would not be so if without prejudice to the natural law a man could be united to a woman otherwise than by marriage. Therefore it is against the natural law to have a concubine.
I answer that, As stated above (A[1]), an action is said to be against the natural law, if it is not in keeping with the due end intended by nature, whether through not being directed thereto by the action of the agent, or through being directed thereto by the action of the agent, or through being in itself improportionate to that end. Now the end which nature intends in sexual union is the begetting and rearing of the offspring. and that this good might be sought after, it attached pleasure to the union; as Augustine says (De Nup. et Concup. i, 8). Accordingly to make use of sexual intercourse on account of its inherent pleasure, without reference to the end for which nature intended it, is to act against nature, as also is it if the intercourse be not such as may fittingly be directed to that end. And since, for the most part, things are denominated from their end, as being that which is of most consequence to them, just as the marriage union took its name from the good of the offspring [*Cf. Q[44], A[2]], which is the end chiefly sought after in marriage, so the name of concubine is expressive of that union where sexual intercourse is sought after for its own sake. Moreover even though sometimes a man may seek to have offspring of such an intercourse, this is not befitting to the good of the offspring, which signifies not only the begetting of children from which they take their being, but also their rearing and instruction, by which means they receive nourishment and learning from their parents, in respect of which three things the parents are bound to their children, according to the Philosopher (Ethic. viii, 11,12). Now since the rearing and teaching of the children remain a duty of the parents during a long period of time, the law of nature requires the father and mother to dwell together for a long time, in order that together they may be of assistance to their children. Hence birds that unite together in rearing their young do not sever their mutual fellowship from the time when they first come together until the young are fully fledged. Now this obligation which binds the female and her mate to remain together constitutes matrimony. Consequently it is evident that it is contrary to the natural law for a man to have intercourse with a woman who is not married to him, which is the signification of a concubine.
Reply to Objection 1: Among the Gentiles the natural law was obscured in many points: and consequently they did not think it wrong to have intercourse with a concubine, and in many cases practiced fornication as though it were lawful, as also other things contrary to the ceremonial laws of the Jews, though not contrary to the law of nature. Wherefore the apostles inserted the prohibition of fornication among that of other ceremonial observances, because in both cases there was a difference of opinion between Jews and Gentiles.
Reply to Objection 2: This law was the result of the darkness just mentioned, into which the Gentiles had fallen, by not giving due honor to God as stated in Rom. 1:21, and did not proceed from the instinct of the natural law. Hence, when the Christian religion prevailed, this law was abolished.
Reply to Objection 3: In certain cases no evil results ensue if a person surrenders his right to a thing whether absolutely or for a time, so that in neither case is the surrender against the natural law. But that does not apply to the case in point, wherefore the argument does not prove.
Reply to Objection 4: Injury is opposed to justice. Now the natural law forbids not only injustice, but also whatever is opposed to any of the virtues: for instance it is contrary to the natural law to eat immoderately, although by doing so a man uses his own property without injury to anyone. Moreover although a bondswoman is her master's property that she may serve him, she is not his that she may be his concubine. And again it depends how a person makes use of his property. For such a man does an injury to the offspring he begets, since such a union is not directed to its good, as stated above.
Reply to Objection 5: The wife has power of her husband's body, not simply and in all respects, but only in relation to marriage, and consequently she cannot transfer her husband's body to another to the detriment of the good of marriage.
Whether it is a mortal sin to have intercourse with a concubine?
Objection 1: It would seem that it is not a mortal sin to have intercourse with a concubine. For a lie is a greater sin than simple fornication: and a proof of this is that Juda, who did not abhor to commit fornication with Thamar, recoiled from telling a lie, saying ( Gn. 38:23 ): "Surely she cannot charge us with a lie." But a lie is not always a mortal sin. Neither therefore is simple fornication.
Objection 2: Further, a deadly sin should be punished with death. But the Old Law did not punish with death intercourse with a concubine, save in a certain case ( Dt. 22:25 ). Therefore it is not a deadly sin.
Objection 3: Further, according to Gregory (Moral. xxxiii, 12), the sins of the flesh are less blameworthy than spiritual sins. Now pride and covetousness, which are spiritual sins, are not always mortal sins. Therefore fornication, which is a sin of the flesh, is not always a mortal sin.
Objection 4: Further, where the incentive is greater the sin is less grievous, because he sins more who is overcome by a lighter temptation. But concupiscence is the greatest incentive to lust. Therefore since lustful actions are not always mortal sins, neither is simple fornication a mortal sin.
On the contrary, Nothing but mortal sin excludes from the kingdom of God. But fornicators are excluded from the kingdom of God ( 1 Cor. 6:9 , 10 ). Therefore simple fornication is a mortal sin.
Further, mortal sins alone are called crimes. Now all fornication is a crime according to Tob. 4:13, "Take heed to keep thyself . . . from all fornication, and beside thy wife never endure to know crime." Therefore, etc.
I answer that, As we have already stated (Sent. ii, D, 42, Q[1], A[4]), those sins are mortal in their genus which violate the bond of friendship between man and God, and between man and man; for such sins are against the two precepts of charity which is the life of the soul. Wherefore since the intercourse of fornication destroys the due relations of the parent with the offspring that is nature's aim in sexual intercourse, there can be no doubt that simple fornication by its very nature is a mortal sin even though there were no written law.
Reply to Objection 1: It often happens that a man who does not avoid a mortal sin, avoids a venial sin to which he has not so great an incentive. Thus, too, Juda avoided a lie while he avoided not fornication. Nevertheless that would have been a pernicious lie, for it would have involved an injury if he had not kept his promise.
Reply to Objection 2: A sin is called deadly, not because it is punished with temporal, but because it is punished with eternal death. Hence also theft, which is a mortal sin, and many other sins are sometimes not punished with temporal death by the law. The same applies to fornication.
Reply to Objection 3: Just as not every movement of pride is a mortal sin, so neither is every movement of lust, because the first movements of lust and the like are venial sins, even sometimes marriage intercourse. Nevertheless some acts of lust are mortal sins, while some movements of pride are venial: since the words quoted from Gregory are to be understood as comparing vices in their genus and not in their particular acts.
Reply to Objection 4: A circumstance is the more effective in aggravating a sin according as it comes nearer to the nature of sin. Hence although fornication is less grave on account of the greatness of its incentive, yet on account of the matter about which it is, it has a greater gravity than immoderate eating, because it is about those things which tighten the bond of human fellowship, as stated above. Hence the argument does not prove.
Whether it was ever lawful to have a concubine?
Objection 1: It would seem that it has been sometimes lawful to have a concubine. For just as the natural law requires a man to have but one wife, so does it forbid him to have a concubine. Yet at times it has been lawful to have several wives. Therefore it has also been lawful to have a concubine.
Objection 2: Further, a woman cannot be at the same time a slave and a wife; wherefore according to the Law ( Dt. 21:11, seqq.) a bondswoman gained her freedom by the very fact of being taken in marriage. Now we read that certain men who were most beloved of God, for instance Abraham and Jacob, had intercourse with their bondswomen. Therefore these were not wives, and consequently it was sometime lawful to have a concubine.
Objection 3: Further, a woman who is taken in marriage cannot be cast out, and her son should have a share in the inheritance. Yet Abraham sent Agar away, and her son was not his heir ( Gn. 21:14 ). Therefore she was not Abraham's wife.
On the contrary, Things opposed to the precepts of the decalogue were never lawful. Now to have a concubine is against a precept of the decalogue, namely, "Thou shalt not commit adultery." Therefore it was never lawful.
Further, Ambrose says in his book on the patriarchs (De Abraham i, 4): "What is unlawful to a wife is unlawful to a husband." But it is never lawful for a wife to put aside her own husband and have intercourse with another man. Therefore it was never lawful for a husband to have a concubine.