Outside observers of Catholicism are often under the mistaken impression that the Catholic Church opposes contraception because She desires Her followers to breed as many children as physically possible.
They are completely wrong. That is not the view of the Catholic Church. That is actually the view of Charles Darwin.
The Catholic Church opposes contraception because She teaches the twofold purpose of sexual intercourse – procreation and unity. Sexual intercourse is meant to unite man and woman together in a joyous, fruitful act that quite literally has the potential to create new life. In the Catholic tradition, this twofold purpose of the sexual act cannot be separated - a couple cannot lawfully have sex solely with the end of pleasure (or, for that matter, solely with the end of baby-making) in mind.
But for Catholics, while every act of sexual intercourse must be open to life, that does not mean that every act of intercourse must be performed with the sole intention of procreation. Indeed, the Catholic Church teaches that men and women can abstain from sexual intercourse during a woman's fertile periods for grave reasons (for example, medical reasons) - a practice known as natural family planning. The Church does NOT teach that couples must have as many children as physically possible.
By contrast, Darwinian evolution postulates that sexual activity evolved over time as a method for individuals to pass on their genetic code to their descendants. In Darwinian evolution, sexual intercourse is for the purpose of reproduction, and the pleasure inherent in the reproductive act merely serves as an incentive for couples to reproduce.
According to Darwinian theory, over time, stronger individuals are better able to pass on their genes and have a greater influence on the gene pool of a species, while weaker individuals are less able to pass on their genetic code, and thus have less influence upon the future of a species.
Having more descendants is a sign of evolutionary success - more descendants means more opportunity to further one's influence on the gene pool. Those who fail to reproduce and pass on their genetic code to descendants are evolutionary failures, because they do not have a chance to pass on their genes to offspring and have no genetic influence on future members of their species.
Thus, humans who have many children are evolutionary successes, in Darwinian terms, because they are able to spread their genetic code to more descendants, and have a greater influence on the gene pool of the human race. By contrast, individuals who contracept thwart their reproductive capacity and deny themselves the chance to influence the human gene pool.
So from a Darwinian standpoint, faithful Catholic married couples who have many children are highly successful, and those who contracept to the point of having no children are Darwinian failures.
Yet despite their insistence on "science" and Darwin's rightness, modern secularists view the act of sexual intercourse solely through the prisms of pleasure and power. They pay homage to Darwin while ignoring his most fundamental principles and scorning basic biology.
The spectacle of secularists who enshrine Darwin and attack the Church for being unscientific, while themselves blithely ignoring basic evolutionary principles, is incredibly ironic - the devotees of reason themselves willfully ignore logic.