St. Augustine argued that sex with any sort of contraceptive intent was a serious sin. Married couples should simply stop having sex at all once they’d had a couple of children. Beyond this statement, he seems not to have touched on the issue. Similarly, the Penitentials of the next several centuries didn’t really focus on or specifically mention contraception much. Instead, as a rule, they tended to discourage any sexual position that didn’t have a chance of resulting in a pregnancy as sinful in and of themselves.
The 11th and 12th centuries saw rising concern with what was “natural” and what wasn’t. St. Peter Damian considered most forms of contraceptive sex to be “against nature.” A bit more than a century later, Gratian called it “inappropriate use of sex organs,” but also considered such things only mildly sinful when the people involved were married to each other.
The 12th century was also when legal and religious writers began paying more attention to the issue. A few people argued that both the use of contraception and sex with contraceptive intent invalidated a marriage entirely. Others said that while marriages entered into with the intent to avoid having children were illegal, they were still binding. Some compared the use of contraception to adultery, while many considered it serious spiritual pollution at the very least.
The thing is, though, for all that several writers had opinions on the topic, contraception wasn’t considered seriously as a separate issue until the 16th century. It was mostly a minor point among far greater concerns.
Brundage, James A. Law, Sex and Christian Society in Medieval Europe. Chicago: University of Chicago Press, 1987.
Birth Control and Abortion in the Middle Ages - Medievalists.net