This thesis examines Las Siete Partidas, a thirteenth-century Castilian legal code of laws, including on marriage and illicit sexual behaviors. Within the scope of medieval marital studies, there are few academic works focused on medieval Iberia in English. This study on the regulation of licit and illicit sexual conduct will analyze the embedded social expectations and stereotypes placed on both sexes. To establish a working definition of this social construct, a pan-European survey of contemporary theological studies on medieval sexuality and marital theories will place the Castilian law into perspective. This definition will be applied to Las Siete Partidas to establish the congruency of the Castilian laws versus other regions of the Medieval Europe. This thesis finds that licit and illicit sexuality shared a symbiotic relationship, where the licit served as a foundation to formulate the illicit. While the Castilian outlook on marriage and illicit sexuality were similar to practices in western Europe, regional variations could be detected in Castilian jurists' synthesis of marital dogma and the particular needs of the community. Las Siete Partidas can thus be seen as the product of a vibrant cultural exchange and an interpretation between the clergy and the laity that challenged the stereotypical image of a culturally stagnant Middle Ages.