Objection 1. It would seem that it is not against the natural law to have several wives. For custom does not prejudice the law of nature. But "it was not a sin" to have several wives "when this was the custom," according to Augustine (De Bono Conjug. xv) as quoted in the text (Sent. iv, D, 33). Therefore it is not contrary to the natural law to have several wives.
Objection 2. Further, whoever acts in opposition to the natural law, disobeys a commandment, for the law of nature has its commandments even as the written law has. Now Augustine says (De Bono Conjug. xv; De Civ. Dei xv, 38) that "it was not contrary to a commandment" to have several wives, "because by no law was it forbidden." Therefore it is not against the natural law to have several wives.
Objection 3. Further, marriage is chiefly directed to the begetting of offspring. But one man may get children of several women, by causing them to be pregnant. Therefore It is not against the natural law to have several wives.
Objection 4. Further, "Natural right is that which nature has taught all animals," as stated at the beginning of the Digests (1, i, ff. De just. et jure). Now nature has not taught all animals that one male should be united to but one female, since with many animals the one male is united to several females. Therefore it is not against the natural law to have several wives.
Objection 5. Further, according to the Philosopher (De Gener. Animal. i, 20), in the begetting of offspring the male is to the female as agent to patient, and as the craftsman is to his material. But it is not against the order of nature for one agent to act on several patients, or for one craftsman to work in several materials. Therefore neither is it contrary to the law of nature for one husband to have many wives.
Objection 6.On the contrary, That which was instilled into man at the formation of human nature would seem especially to belong to the natural law. Now it was instilled into him at the very formation of human nature that one man should have one wife, according to Genesis 2:24, "They shall be two in one flesh." Therefore it is of natural law.
Objection 7. Further, it is contrary to the law of nature that man should bind himself to the impossible, and that what is given to one should be given to another. Now when a man contracts with a wife, he gives her the power of his body, so that he is bound to pay her the debt when she asks. Therefore it is against the law of nature that he should afterwards give the power of his body to another, because it would be impossible for him to pay both were both to ask at the same time.
Objection 8. Further, "Do not to another what thou wouldst not were done to thyself" [Cf. Tobit 4:16 is a precept of the natural law. But a husband would by no means be willing for his wife to have another husband. Therefore he would be acting against the law of nature, were he to have another wife in addition.
Objection 9. Further, whatever is against the natural desire is contrary to the natural law. Now a husband's jealousy of his wife and the wife's jealousy of her husband are natural, for they are found in all. Therefore, since jealousy is "love impatient of sharing the beloved," it would seem to be contrary to the natural law that several wives should share one husband.
I answer that, All natural things are imbued with certain principles whereby they are enabled not only to exercise their proper actions, but also to render those actions proportionate to their end, whether such actions belong to a thing by virtue of its generic nature, or by virtue of its specific nature: thus it belongs to a magnet to be borne downwards by virtue of its generic nature, and to attract iron by virtue of its specific nature. Now just as in those things which act from natural necessity the principle of action is the form itself, whence their proper actions proceed proportionately to their end, so in things which are endowed with knowledge the principles of action are knowledge and appetite. Hence in the cognitive power there needs to be a natural concept, and in the appetitive power a natural inclination, whereby the action befitting the genus or species is rendered proportionate to the end. Now since man, of all animals, knows the aspect of the end, and the proportion of the action to the end, it follows that he is imbued with a natural concept, whereby he is directed to act in a befitting manner, and this is called "the natural law" or "the natural right," but in other animals "the natural instinct." For brutes are rather impelled by the force of nature to do befitting actions, than guided to act on their own judgment. Therefore the natural law is nothing else than a concept naturally instilled into man, whereby he is guided to act in a befitting manner in his proper actions, whether they are competent to him by virtue of his generic nature, as, for instance, to beget, to eat, and so on, or belong to him by virtue of his specific nature, as, for instance, to reason and so forth. Now whatever renders an action improportionate to the end which nature intends to obtain by a certain work is said to be contrary to the natural law. But an action may be improportionate either to the principal or to the secondary end, and in either case this happens in two ways. First, on account of something which wholly hinders the end; for instance a very great excess or a very great deficiency in eating hinders both the health of the body, which is the principal end of food, and aptitude for conducting business, which is its secondary end. Secondly, on account of something that renders the attainment of the principal or secondary end difficult, or less satisfactory, for instance eating inordinately in respect of undue time. Accordingly if an action be improportionate to the end, through altogether hindering the principal end directly, it is forbidden by the first precepts of the natural law, which hold the same place in practical matters, as the general concepts of the mind in speculative matters. If, however, it be in any way improportionate to the secondary end, or again to the principal end, as rendering its attainment difficult or less satisfactory, it is forbidden, not indeed by the first precepts of the natural law, but by the second which are derived from the first even as conclusions in speculative matters receive our assent by virtue of self-known principles: and thus the act in question is said to be against the law of nature.
Now marriage has for its principal end the begetting and rearing of children, and this end is competent to man according to his generic nature, wherefore it is common to other animals (Ethic. viii, 12), and thus it is that the "offspring" is assigned as a marriage good. But for its secondary end, as the Philosopher says (Ethic. viii, 12), it has, among men alone, the community of works that are a necessity of life, as stated above (Question 41, Article 1). And in reference to this they owe one another "fidelity" which is one of the goods of marriage. Furthermore it has another end, as regards marriage between believers, namely the signification of Christ and the Church: and thus the "sacrament" is said to be a marriage good. Wherefore the first end corresponds to the marriage of man inasmuch as he is an animal: the second, inasmuch as he is a man; the third, inasmuch as he is a believer. Accordingly plurality of wives neither wholly destroys nor in any way hinders the first end of marriage, since one man is sufficient to get children of several wives, and to rear the children born of them. But though it does not wholly destroy the second end, it hinders it considerably for there cannot easily be peace in a family where several wives are joined to one husband, since one husband cannot suffice to satisfy the requisitions of several wives, and again because the sharing of several in one occupation is a cause of strife: thus "potters quarrel with one another" [Aristotle, Rhet. ii, 4, and in like manner the several wives of one husband. The third end, it removes altogether, because as Christ is one, so also is the Church one. It is therefore evident from what has been said that plurality of wives is in a way against the law of nature, and in a way not against it.
Reply to Objection 1. Custom does not prejudice the law of nature as regards the first precepts of the latter, which are like the general concepts of the mind in speculative matters. But those which are drawn like conclusions from these custom enforces, as Tully declares (De Inv. Rhet. ii), or weakens. Such is the precept of nature in the matter of having one wife.
Reply to Objection 2. As Tully says (De Inv. Rhet. ii), "fear of the law and religion have sanctioned those things that come from nature and are approved by custom." Wherefore it is evident that those dictates of the natural law, which are derived from the first principles as it were of the natural law, have not the binding force of an absolute commandment, except when they have been sanctioned by Divine or human law. This is what Augustine means by saying that "they did not disobey the commandments of the law, since it was not forbidden by any law."
The Reply to the Third Objection follows from what has been said.
Reply to Objection 4. Natural right has several significations. First a right is said to be natural by its principle, because it is instilled by nature: and thus Tully defines it (De Inv. Rhet. ii) when he says: "Natural right is not the result of opinion but the product of an innate force." And since even in natural things certain movements are called natural, not that they be from an intrinsic principle, but because they are from a higher moving principle--thus the movements that are caused in the elements by the impress of heavenly bodies are said to be natural, as the Commentator states (De Coelo et Mundo iii, 28), therefore those things that are of Divine right are said to be of natural right, because they are caused by the impress and influence of a higher principle, namely God. Isidore takes it in this sense, when he says (Etym. v) that "the natural right is that which is contained in the Law and the Gospel." Thirdly, right is said to be natural not only from its principle but also from its matter, because it is about natural things. And since nature is contradistinguished with reason, whereby man is a man, it follows that if we take natural right in its strictest sense, those things which are dictated by natural reason and pertain to man alone are not said to be of natural right, but only those which are dictated by natural reason and are common to man and other animals. Thus we have the aforesaid definition, namely: "Natural right is what nature has taught all animals." Accordingly plurality of wives, though not contrary to natural right taken in the third sense, is nevertheless against natural right taken in the second sense, because it is forbidden by the Divine law. It is also against natural right taken in the first sense, as appears from what has been said, for such is nature's dictate to every animal according to the mode befitting its nature. Wherefore also certain animals, the rearing of whose offspring demands the care of both, namely the male and female, by natural instinct cling to the union of one with one, for instance the turtle-dove, the dove, and so forth.
The Reply to the Fifth Objection is clear from what has been said.
Since, however, the arguments adduced "on the contrary side" would seem to show that plurality of wives is against the first principles of the natural law, we must reply to them.
Accordingly we reply to the Sixth Objection that human nature was founded without any defect, and consequently it is endowed not only with those things without which the principal end of marriage is impossible of attainment, but also with those without which the secondary end of marriage could not be obtained without difficulty: and in this way it sufficed man when he was first formed to have one wife, as stated above.
Reply to Objection 7. In marriage the husband gives his wife power of his body, not in all respects, but only in those things that are required by marriage. Now marriage does not require the husband to pay the debt every time his wife asks for it, if we consider the principal end for which marriage was instituted, namely the good of the offspring, but only as far as is necessary for impregnation. But in so far as it is instituted as a remedy (which is its secondary end), marriage does require the debt to be paid at all times on being asked for. Hence it is evident that by taking several wives a man does not bind himself to the impossible, considering the principal end of marriage; and therefore plurality of wives is not against the first principles of the natural law.
Reply to Objection 8. This precept of the natural law, "Do not to another what thou wouldst not were done to thyself," should be understood with the proviso that there be equal proportion. For if a superior is unwilling to be withstood by his subject, he is not therefore bound not to withstand his subject. Hence it does not follow in virtue of this precept that as a husband is unwilling for his wife to have another husband, he must not have another wife: because for one man to have several wives is not contrary to the first principles of the natural law, as stated above: whereas for one wife to have several husbands is contrary to the first principles of the natural law, since thereby the good of the offspring which is the principal end of marriage is, in one respect, entirely destroyed, and in another respect hindered. For the good of the offspring means not only begetting, but also rearing. Now the begetting of offspring, though not wholly voided (since a woman may be impregnated a second time after impregnation has already taken place, as stated in De Gener. Animal. vii. 4), is nevertheless considerably hindered, because this can scarcely happen without injury either to both fetus or to one of them. But the rearing of the offspring is altogether done away, because as a result of one woman having several husbands there follows uncertainty of the offspring in relation to its father, whose care is necessary for its education. Wherefore the marriage of one wife with several husbands has not been sanctioned by any law or custom, whereas the converse has been.
Reply to Objection 9. The natural inclination in the appetitive power follows the natural concept in the cognitive power. And since it is not so much opposed to the natural concept for a man to have several wives as for a wife to have several husbands, it follows that a wife's love is not so averse to another sharing the same husband with her, as a husband's love is to another sharing the same wife with him. Consequently both in man and in other animals the male is more jealous of the female than "vice versa."
Source: Summa Theologica (New Advent)