Final answer:
In Yanomami society, which practices unilineal descent, sex with parallel cousins is taboo due to close lineage ties, while sex with cross cousins is acceptable because they belong to different lineages, creating potential for marital alliances.
Step-by-step explanation:
Among the Yanomami, a society with unilineal descent, sex with cross cousins is considered appropriate, whereas sex with parallel cousins is seen as incestuous. The reason for this is found in the cultural understanding of kinship and descent patterns. In unilineal descent societies, kinship is traced through only one gender line, either male or female. This creates two distinct types of cousins: parallel and cross. Parallel cousins are the children of a person's parent's same-sex sibling (your father's brother's children or your mother's sister's children), whereas cross cousins are the children of a person's parent's opposite-sex siblings (your father's sister's children or your mother's brother's children).
In many societies with unilineal descent, such as the Yanomami, parallel cousins are considered too closely related for sexual relationships because they fall within the same lineage or clan. These kinship ties are typically accompanied by rights and responsibilities that reinforce the incest taboo. On the other hand, cross cousins are considered acceptable marital partners because they belong to different lineages or clans, and this allows for alliances to be formed between different family groups. In essence, sex with parallel cousins is viewed as incestuous as they are considered actual kin, while cross cousins are viewed as potential affinal ties or marital partners.