Invalidating Marriages. The medieval Church identified two types of impediments to marriage. Diriment impediments blocked the creation of the marriage bond. If two people exchanged consent and a diriment impediment were discovered, the couple was required to separate immediately because no valid marriage bond existed—nor could ever exist—between them. The second type was the prohibitive impediment. Though this second impediment forbade a marriage between two individuals, if they married anyway, their union was considered valid and indissoluble.

Consanguinity. Consanguinity, or blood relationship, was the most significant and most ancient reason for prohibiting a union. Most societies throughout history have forbidden marriage between siblings or relatives in the direct line, such as grandfather and granddaughter. In ancient Rome, aunts and uncles could not marry their nieces and nephews, but first cousins were permitted to marry...