Argument: Contraception undermines sexual morality; encourages promiscuity, infidelity
Humanae Vitae, Pope Paul VI - "Consequences of Artificial Methods
17. Responsible men can become more deeply convinced of the truth of the doctrine laid down by the Church on this issue if they reflect on the consequences of methods and plans for artificial birth control. Let them first consider how easily this course of action could open wide the way for marital infidelity and a general lowering of moral standards. Not much experience is needed to be fully aware of human weakness and to understand that human beings—and especially the young, who are so exposed to temptation—need incentives to keep the moral law, and it is an evil thing to make it easy for them to break that law."
Benedict XVI said in 2005, that contraception leads to a “breakdown in sexual morality.”