In medieval times the Church faced the daunting and difficult task of convincing society that marriage, once considered a family affair, must be truly monogamous and conform to certain Christian norms.
One of the norms that the Church tried to impose in the early Middle Ages was exogamy or prohibition of incest. The extension of the taboo of incest even to remote relatives is a unique feature of Christian legislation of the early Middle Ages. There was no precedent for such a wide prohibition in classical Roman law that limited the taboo to very close relatives.
The struggle against incest intensified in the sixth century. In Gaul, councils threatened the incestuous with excommunication. Civil law also imposed such prohibitions. At first, the Merovingians were reluctant to follow the Church in this matter-Clotario and Cariberto were remarried with their sisters-in-law, but in 630 Dagobert II punished the incestors with the confiscation of their property. The prohibitions were sensibly extended in the eighth century. The Council of Rome in the year 721 excluded all consanguineous relatives and "spiritual relatives", that is, the godparents. Under Charlemagne, the number of councils on this subject multiplied, and in the ninth century, the ban was extended to sixth-grade cousins.
Why did the Church place so much emphasis on these prohibitions that they had no precedent even in the Bible? Some historians see in this struggle against incest a real conspiracy of the Church devised to reduce the number of heirs and thus multiply the number of cases in which the inheritance could pass to the Church. Others, on the contrary, believe that the Church was not centralized enough to impose such a policy in a unified way.
There are also those who believe that the purpose of these prohibitions was to ensure harmonious family and social relations. It could be added to this explanation that the prohibitions of incest are comparable to the numerous taboos related to sex (and sometimes to blood) which were preached by the Church of the Early Medieval with the intention of reducing the occasions in which the sexual intercourse, even among married couples. Numerous canons prohibited sexual intercourse with menstruating women and condemned the sexual act on holidays.
The Church managed to convince the Christian population to respect such taboos by threatening divine punishments, such as the birth of a deformed son conceived in a forbidden sexual act. Such fears could also be caused by waving the red incest flag. Charlemagne, for example, who had possibly committed incest with his sister, interpreted the death of Roldan - who would have been his son and nephew - as a punishment for his sin.
The civil authorities also had reasons that were not ecclesiastical to cooperate in the fight against incest. The endogamy of the aristocracy was a threat to the state because the concentration of land and wealth in the hands of large family groups could weaken public authority and threaten civil peace. The legal theory underlying the prohibitions of incest was perfected in the twelfth century in the context of the development of a coherent theory of marriage. Consanguinity and spiritual kinship became impediments to marriage, and although two people were married the marriage was void, and the couple had to separate.