Notice carefully that none of the arguments below are related to genetics or biology, which is important because some wag will always claim it is possible to “cure” genetic deficiencies caused by, say, first cousin marriages.
1 Moreover, because of reasonable considerations of this kind it has been ordered by the laws that certain persons, related by their origin, are excluded from matrimony.
2 In fact, since there is in matrimony a union of diverse persons, those persons who should already regard themselves as one because of having the same origin are properly excluded from matrimony, so that in recognizing themselves as one in this way they may love each other with greater fervor.
3 Again, because the acts performed by husband and wife are associated with a certain natural shame, it is necessary that those persons to whom respect is due because of the bond of blood should be prohibited from performing such actions with each other. Indeed, this reason seems to have been suggested in the Old Testament law, in the text which states: “You shall not uncover the nakedness of your sister” (Lev. 18:9), and also in other texts.
4 Besides, for man to be much given to sexual pleasures contributes to the dissolution of good moral behavior; because, since this pleasure greatly occupies the mind, reason is withdrawn from things which should be done rightly. Now, if a man were permitted sexual relations with those persons with whom he must live, such as sisters and other relatives, excessive indulgence in this pleasure would result, for the occasion for sexual relations with such persons could not be removed. Therefore, it was suitable to good moral behavior for such union to be prohibited by laws.
5 Furthermore, the enjoyment of sexual relations “greatly corrupts the judgment of prudence.” So, the multiplication of such pleasure is opposed to good behavior. Now, such enjoyment is increased through the love of the persons who are thus united. Therefore, intermarriage between relatives would be contrary to good behavior, for, in their case, the love which springs from community of origin and upbringing would be added to the love of concupiscence, and, with such an increase of love, the soul would necessarily become more dominated by these pleasures.
Notes Make that excessive enjoyment.
6 Moreover, in human society it is most necessary that there be friendship among many people. But friendship is increased among men when unrelated persons are bound together by matrimony. Therefore, it was proper for it to be prescribed by laws that matrimony should be contracted with persons outside one’s family and not with relatives.
7 Besides, it is unfitting for one to be conjugally united with persons to whom one should naturally be subject. But it is natural to be subject to one’s parents. Therefore, it would not be fitting to contract matrimony with one’s parents, since in matrimony there is a conjugal union.
8 Hence it is said: “No man shall approach to her that is near of kin to him” (Lev. 18:6).
9 By these arguments the custom of those who practice carnal relations with their relatives is refuted.
10 Moreover, we should note that just as natural inclination tends toward things which happen in most cases, so also positive law depends on what happens in most cases. It is not contrary to the foregoing arguments if in a particular case the outcome might be otherwise, for the good of many should not be sacrificed for the sake of one person’s good, because “the good of many is always more divine than the good of one person.”
However, lest the disadvantage which could occur in the individual case be altogether without remedy, there remains with lawmakers and others of similar function the authority to grant a dispensation from what is generally required by law, in view of what is necessary in any particular case. For, if the law be a human one, it can be dispensed by men who have such power. But, if the law be divinely given, dispensation can be granted by divine authority; as, in the Old Law, permission seems to have been granted by dispensation to have several wives and concubines and to put away one’s wife.