Specifically, religious beliefs that one ineluctable purpose of sex is to procreate - so that engaging in sex while deliberately preventing the possibility of conception is a moral violation. It violates what they believe is G-d's command to "be fruitful and multiply."

"Rhythm method" prevents procreation, without chemical or physical barriers. Is it "artificial"? What is the difference between rhythm and abstinence? Abstinence is a conscious decision to avoid procreation. Anything that prevents procreation is against "God's will".

How long have politics and religion been intertwined? Since about forever. Traditionally, the crown has also been "defender/protector of the faith". In return, the religion supports the crown, with the "divine right of kings" narrative. What does the crown want? More money and power. How does the crown obtain more money and power? 1-more workers to make more stuff, that the crown owns. 2-more soldiers to fight the crown's wars to take wealth and power from others, for the benefit of the crown. The crown wants it's Proles to breed like rabbits, because more Proles result in more wealth and power for the crown, so the religion the crown "protects/defends" adjusts it's doctrine to support what the crown wants, lest it be replaced by "new religion" that does give the crown what it wants (see Henry VIII). Then the people bow down to the crown's religion, lest they be burned at the stake for heresy. (see the religious kangaroo court the English crown arranged to get rid of Joan)

From the net sifter:

Yes — the Spanish Inquisition was not an independent papal institution; it was created and operated under the direct authority of the Spanish Crown.

When Pope Sixtus IV granted permission in 1478 to establish the Inquisition in Spain, it was done at the request of Ferdinand II of Aragon and Isabella I of Castile, the Catholic Monarchs who had just unified their kingdoms. Unlike the medieval papal Inquisition, which was controlled by the Church, the Spanish Inquisition was a hybrid religious–secular body. The Crown appointed its inquisitors, oversaw its operations, and could replace or remove officials

Steve