Summa contra Gentiles3.125 Chapter CXXV. That marriage should not be contracted between relatives

For these reasonable motives the laws have decreed that certain persons belonging to a common stock should be debarred from marriage.

Because, as marriage is the union of different persons, those who should consider themselves as one through having a common ancestor, are rightly debarred from marriage, that they may have a greater regard for each other, through realizing that for that reason they are one.

Again. Since in the relations between husband and wife there is a certain natural shame, relations should be forbidden between those who through being united in blood should revere each other. This motive seems to be indicated in the law, where it is said: Thou shalt not uncovey the nakedness of thy sister, and so on.

Further. That men be too much given to the pleasure of copulation is corruptive of good morals: because, since, more than any other, this pleasure absorbs the mind, the reason would be hindered in things pertaining to rectitude. Now there would result an abuse of pleasure if man were allowed to be united in copulation with those, in whose society he must needs live, such as sisters and other relatives: for it would be impossible to remove the occasion of intercourse with such persons. It was therefore in keeping with good morals that the laws should forbid such unions.

Moreover. The pleasure of copulation corrupts entirely the judgement of prudence. Therefore frequency of that pleasure is contrary to good morals. Now, this pleasure is increased by the mutual love of those who are thus united. Hence it would be contrary to good morals for relatives to marry: for then there would be in them the love arising from community of blood and nourishment in addition to the love of desire; and, in consequence, through multiplicity of loves the soul would be all the more a slave to pleasures.

Besides. It is most necessary in human society that friendship be among many. Now friendships among men are multiplied if marriage be contracted between persons of different stock. Therefore it was becoming for the laws to direct that marriage be contracted with those of different stock, and not between relatives.

Also. It is unsuitable that a person be united socially with those to whom he should naturally be subject. Now, it is natural for a man to be subject to his parents. Therefore it is unfitting for one to marry one’s parent, since marriage is a social union.

Hence it is said (Levit. xviii. 6): No man shall approach to her that is near of kin to him.

Hereby stands condemned the custom of those who contract bonds of the flesh with persons of their kindred. We must observe, however, that just as the natural inclination is to what occurs most frequently, so too is the law made to fit the majority of cases. The foregoing arguments are not invalidated by any possible exceptions: for the good of many should not be foregone for the sake of the good of one, since the common good is ever more god-like than the good of one} Lest, however, a defect that may occur in a single instance, be altogether irremediable, lawgivers and the like have the power to dispense in the statutes that are made for the generality, according to the requirements of a particular case. If the law be made by man, those who have the same power can dispense therein. But if the law be of God, dispensation can be granted by divine authority: just as in the old law polygamy, concubinage and divorce were seemingly allowed by dispensation.