1 It seems, too, that we should consider how it is inborn in the minds of all animals accustomed to sexual reproduction to allow no promiscuity; hence, fights occur among animals over the matter of sexual reproduction. And, in fact, among all animals there is one common reason, for every animal desires to enjoy freely the pleasure of the sexual act, as he also does the pleasure of food; but this liberty is restricted by the fact that several males may have access to one female, or the converse.
The same situation obtains in the freedom of enjoying food, for one animal is obstructed if the food which he desires to eat is taken over by another animal. And so, animals fight over food and sexual relations in the same way. But among men there is a special reason, for, as we said, man naturally desires to know his offspring, and this knowledge would be completely destroyed if there were several males for one female. Therefore, that one female is for one male is a consequence of natural instinct.
2 But a difference should be noted on this point. As far as the view that one woman should not have sexual relations with several men is concerned, both the aforementioned reasons apply. But, in regard to the conclusion that one man should not have relations with several females, the second argument does not work, since certainty as to offspring is not precluded if one male has relations with several women.
But the first reason works against this practice, for, just as the freedom of associating with a woman at will is taken away from the husband, when the woman has another husband, so, too, the same freedom is taken away from a woman when her husband has several wives. Therefore, since certainty as to offspring is the principal good which is sought in matrimony, no law or human custom has permitted one woman to be a wife for several husbands. This was even deemed unfitting among the ancient Romans, of whom Maximus Valerius reports that they believed that the conjugal bond should not be broken even on account of sterility.
3 Again, in every species of animal in which the father has some concern for offspring, one male has only one female; this is the case with all birds that feed their young together, for one male would not be able to offer enough assistance to bring up the offspring of several females. But in the case of animals among whom there is no concern on the part of the males for their offspring, the male has promiscuous relations with several females and the female with plural males. This is so among dogs, chickens, and the like. But since, of all animals, the male in the human species has the greatest concern for offspring, it is obviously natural for man that one male should have but one wife, and conversely.
4 Besides, friendship consists in an equality. So, if it is not lawful for the wife to have several husbands, since this is contrary to certainty as to offspring, it would not be lawful, on the other hand, for a man to have several wives, for the friendship of wife for husband would not be free, but somewhat servile. And this argument is corroborated by experience, for among husbands having plural wives the wives have a status like that of servants.
5 Furthermore, strong friendship is not possible in regard to many people, as is evident from the Philosopher in Ethics VIII . Therefore, if a wife has but one husband, but the husband has several wives, the friendship will not be equal on both sides. So, the friendship will not be free, but servile in some way.
6 Moreover, as we said, matrimony among humans should be ordered so as to be in keeping with good moral customs. Now, it is contrary to good behavior for one man to have several wives, for the result of this is discord in domestic society, as is evident from experience. So, it is not fitting for one man to have several wives.
7 Hence it is said: “They shall be two in one flesh” (Gen. 7.: 24).
8 By this, the custom of those having several wives is set aside, and also the opinion of Plato who maintained that wives should be common. And in the Christian period he was followed by Nicolaus, one of the seven deacons.