In medieval Spain before the reign of Ferdinand III (1217-1252), relations between Christians, Muslims and Jews were strained but navigable. They even had a word for this unsteady commingling of society: convivencia. Rules existed to further the cause of Reconquista--the Christian reclamation of the Iberian Peninsula from the Islamic tribes--and those rules changed depending not on any set moral principle, but on the success of re-conquest.
For example, when Christian kings began to make substantial territorial advancements in the 11th century, they employed Jews as spies and advisers. Christians did not become spies because it was thought to be immoral, and also, because Jews had been in the Moorish-occupied territory throughout the Christian expulsion, they knew a great deal more about the Islamic tribes.
Jews had performed the same function in the 8th century when they advised the invading Moors about the habits and weaknesses of the conquered Visigoths. This pattern of adaptation--finding usefulness within an incoming regime--helped the Jews survive multiple invasions and re-conquests, but it added to the suspicions about their people and way of life.
More than 2,000 Jews (7% of the population) lived in Toledo under Alfonso VIII of Castile, and Jews were awarded juderias, or Jewish-controlled estates in the north of the Peninsula. Some were even awarded daughters as political alliances. Interfaith sexual relations were in no way forbidden at this time.
Although Jews enjoyed these advantages in the heyday of Reconquista, the need for their expertise waned as Christians began to re-occupy more and more land and become acquainted first-hand with their Moorish enemies. Eventually, in the early 13th century, Jews were expelled from their juderias, replaced in royal courts, and forced to wear identifying clothing. The taboo of marrying a Jew reappeared in good society, although their integration into Christian bloodlines in Spain was greater than in any other medieval kingdom.
Because the ultimate goal of Reconquista was to take back Iberian lands from the Moors, the most heinous crime a Christian could commit was sexual relations with a Muslim. Islamic women could be put to death for having sex with a Christian, and Christian women with a Muslim. Even prostitutes had to stay with men of their own religion. Men were generally given a warning--the removal of a hand, for example--but they were put to death as well after repeated transgressions.
However, because the need for heirs to secure new, tenuous Christian bloodlines was of paramount interest to the kings, and with Christian women so scarce on the frontier, certain provisions were made for mixed-race children. If a Christian man fathered a baby on a Muslim slave, and then that baby was baptized, the man could name the child as his heir--anything to continue at least nominal control over acquired lands.
This aspect of their society played a key role in the background of Gavriel, the hero of my 2009 release SCOUNDREL'S KISS:
Customary anger shot through Gavriel's limbs. "I learned, for example, that I was kept a slave illegally. For my entire youth I was told that my mother's fate was my own. Servitude. But I was baptized, Ada. I was not instructed in the ways of the Church, but I am Christian."And while maintaining a mistress was not a taboo--in fact, because frontier women were so sought after, mistresses could then go on to proper marriages without any lasting damage to their reputations--a married woman's adultery was an offense publishable by death. The last thing these Christian communities wanted were marriages made unstable by adultery. Unstable marriage meant divorce. Divorce meant bachelor knights--known for their unpredictability, violence, and lack of a "homestead" lifestyle. Itinerancy was not good for reconquest. Maintaining a mistress, although not ideal according to the Church, at least meant bachelors behaved as more stable guardians of frontier society.
Ada gasped. "But slaves are freed if they convert. Isn't that true? You should have been raised a free man!"
"Yes, but I was not," he said with grim resolve. "Now I fear being free of this place. I would rejoin their ranks as a warrior, or I would kill the father who bound me."
I find it amusing that iron-clad rules about who can sleep with whom moved with the times, according to the kings in charge, their aims, and the status of their campaigns against an enemy. Some social taboos are not absolute.