(CG 3/123). That Marriage ought to be Indissoluble (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

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ONE general reason holds for all animals, which is this, that every animal desires free enjoyment of the pleasure of sexual union as of eating: which freedom is impeded by there being either several males to one female, or the other way about: and therefore animals fight alike for food and for sexual jealousy. But in men there is a special reason, inasmuch as man naturally desires to be sure of his own offspring. But here a difference comes in. Both of the above mentioned reasons hold for the case of the cohabitation of one female with several males: but the second reason does not hold against the cohabitation of one male with several females,�I mean certainty in point of parentage is not in that case prevented. But the first reason makes against it: for as the free enjoyment of the female is taken from the male, if the female has another partner, so the same free enjoyment is taken from the female, if the male has more than one partner.

2. In every species of animal in which the sire takes any interest in the offspring, one male keeps company with one female only, as in all birds that rear their young in common: for one male could not avail for several females as a helper in the rearing of their progeny: whereas in animals in which the males take no interest in the offspring, one male consorts with several females promiscuously, and the female with several males, as appears in dogs, poultry, and the like. But the male's interest in the offspring is greater in the human species than in any other.

3 and 4. The reason why a wife is not allowed more than one husband at a time is because otherwise paternity would be uncertain. If then while the wife has one husband only, the husband has more than one wife, there will not be a friendship of equality on both sides, friendship consisting in a certain equality. There will not be the friendship of a free man with a free woman, but a sort of friendship of a slave with her master. The husband might well be allowed a plurality of wives, if the understanding were allowable, that the friendship of each with him was not to be that of a free woman with a free man but of a slave with her master. And this is borne out by experience: for among men that keep many wives the wives are counted as menials.

5. From one man having several wives there arises discord at the domestic hearth, as experience shows.

Hence it is said: They shall be two in one flesh (Gen. ii, 24).