§ 1. Concerning the Unity of Matrimony

8. • I. The unity of matrimony consists in this, that one man is joined to one woman. It excludes both polyandry, i.e., the simultaneous plurality of husbands, and polygamy, namely, the simultaneous plurality of wives, but by no means second or further marriages after the death of the spouse.

II. Simultaneous polyandry is plainly condemned by the natural law, since it is directly contrary to the primary end of matrimony, namely, the generation and education of offspring. For experience teaches that women habitually known by several men are rarely fruitful, and moreover polyandry, by rendering paternity uncertain, would deprive the offspring of the care and assistance of the father.

III. Polygamy is not thus opposed to the natural law, since one man suffices for making several women fruitful and for caring for the education of the offspring received from them. And in fact God granted the patriarchs permission to use polygamy. Nevertheless a plurality of wives does not sufficiently provide for the good of peace among spouses, nor does it preserve the equal right of man and woman in the asking of the conjugal debt. And therefore theologians say that polygamy is opposed to the secondary natural law, whose office is to prescribe those things which are necessary not for simply being, but for being well and better.

Christ restored the matrimonial institution to its first purity and unity, and thereafter it is licit for no man, even a gentile, to have several wives at the same time; and, if such marriages are attempted, they are plainly invalid.