Why does exogamy not guarantee that two spouses are genetically unrelated?

Exogamy is the custom of marrying outside one's own social group, such as a clan or lineage. However, it does not guarantee that spouses are genetically unrelated because these rules are typically defined by social kinship structures rather than biological consanguinity.1

In many societies, unilineal descent systems organize individuals into broad groups. While members of one's own lineage are often excluded as marriage partners, cross-cousins—the children of one's mother's brother (in a matrilineal system) or father's sister (in a patrilineal system)—are often considered ideal marriage partners despite being consanguineous kin who share common ancestors. Because exogamy focuses on maintaining social boundaries rather than preventing the inheritance of shared genes, it is entirely possible for individuals from different social lineages to still share a common ancestor, thereby maintaining a degree of genetic relationship.12