Queen Isabella I of Castile and King Ferdinand II of Aragon were cousins. Under Catholic law, they were not allowed to get married. When they (in consultation with their powerful families and supporters) decided to get married, probably for political reasons because they had not met before their marriage, Isabella was less than eighteen and Ferdinand was sixteen. The Papal representative in Spain, Antonio Veneris, forged a document which made it possible for Ferdinand to marry his cousin. The document which made the marriage legal was signed in January 1469.
With this marriage, Castile and Aragon were united into a formidable kingdom, jointly ruled by Isabella and Ferdinand. During their reign, the Nasrid kingdom of Granada was destroyed, the Reconquista was completed, Spain was aggressively Christianized and united, Columbus was sent on a voyage in which he made the discovery of a new world of fertile territory, gold, and slaves, the Americas, and the foundation of the Spanish Empire was laid. This marriage launched the Age of Imperialism which brought territory, slaves, and wealth to the imperialist nations and resulted in Europe marching far ahead of all other civilizations.
There was a bit of sin involved in his marriage, since Isabella and Ferdinand were within third degree of cousinage. But Pope Alexander VI granted them the title of “Catholic Monarch.” From the point of view of Western history, the marriage of Isabella and Ferdinand is the most important one—it is the West’s marriage of the millennium. Had they not married, then it is possible there would be no Western power today and history could have taken a different course.