IT is also to be observed, seemingly, that all animals that are used to copulate, have a natural instinct to resist another’s intercourse with their consort: wherefore, animals ght on account of copulation. And as regards all animals there is one common reason for this, because every animal desires to indulge at will in the pleasure of copulation, even as in the pleasure of eating: and this freedom ceases if many males have access to one female, or vice versa: just as an animal is deprived of the free enjoyment of its food, if another animal despoil it of the food it desires to consume. Hence animals fight both for food and for copulation. But with regard to men there is a special reason: because . . . man naturally desires to be assured of his offspring: and this assurance would be altogether nullified in the case of promiscuous copulation. Therefore the union of one man with one woman comes from a natural instinct.
A difference, however, is to be noted here. For as regards one woman not being united to several men, both the foregoing arguments avail. But as regards one man not being joined to several women, the second argument is of no use: since the certainty of having offspring is not removed if one man be joined to several women. The first argument, however, avails against this: for just as freedom of access to the woman is denied, if she have another man, so too the same freedom is denied the woman, if the man have several women. Hence, as certainty of having offspring is the chief good sought from marriage, no human law or custom has permitted polyandry. This was considered to be wrong even among the ancient Romans, of whom Maximus Valerius relates that they deemed that not even on account of barrenness should the marriage bond be severed.
Again. In every animal species where the father has a certain care for his offspring, the one male has but one female, as may be seen in birds, where both unite in feeding their young: for one male would not suffice to rear the progeny of several females. On the other hand where the male animal has not the care of the offspring, we find indifferently union of one male with several females, or of one female with several males: such is the case with dogs, hens, and so forth. Since then of all animals the male of the human species is pre-eminent in the care of his offspring, it is clearly natural to man that one man should have one wife and vice versa.
Besides. Equality is a condition of friendship. Hence if a woman may not have several husbands, because this removes the certainty of offspring; were it lawful for a man to have several wives, the friendship of a wife for her husband would not be freely bestowed, but servile as it were. And this argument is confirmed by experience: since where men have several wives, the wives are treated as servants.
Further. In perfect friendship it is impossible to be friends with many, according to the Philosopher (8 Ethic. vi.). Hence if the wife has but one husband, while the husband has several wives, the friendship will not be equal on either side: and consequently it will be not a freely bestowed but a servile friendship as it were.
Moreover. As we have already stated matrimony among men should be so ordered as to be consistent with good morals. Now it is contrary to good morals that one man have several wives, for this leads to discord in the family, as shown by experience. Therefore it is not right for one man to have several wives.
Hence it is said (Gen. ii. 24): They shall be two in one flesh. – from Summa Contra Gentiles, third book: Chapter CXXIV: That matrimony should be the union of one man with one woman.
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