The Church is not saying that every couple has the obligation to have as many children as they can. Although each marital act is open to life, a couple may space births of their children. The reasons for doing so must not be motivated by selfishness. The decision to limit birth should be done with respect to the meaning of self-giving and human procreation in the context of true love. The method for regulating birth must be in accord with human dignity and the love of God. This method is called Natural Family Planning and is the only method that does not contravene God's law. Contraceptive chemicals or devices are in and of themselves sinful because each interferes with God's natural law. The big plus to NFP is that it is completely natural and safe and is as effective as any artificial means. Some older folks may think that this is the rhythm method that they remember from the 1940s and 50s. But, it is not. NFP is a scientific method to determine when the fertile and infertile periods occur in the wife's cycle. It is quite accurate.
Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, "every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" is intrinsically evil:
Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. (The Catechism of the Catholic Church, 2370)
Marriage is God's institution, not man's creation. We must respect its nature as intended by God: a man and a woman totally giving themselves to each other in a union that is open to life, that is, the birth of a child. This union is permanent, until death they do part.
Some of the current practices going on today are wrong. Here are some which are contrary to God;
Any sexual relations outside of marriage, adultery, same sex union (or marriage), surrogate motherhood, artificial insemination, artificial contraception, sterilization, polygamy, abortion, and also, civil divorce of a valid marriage is contrary to God's law. The spouse who is the victim of the divorce is innocent. However, sometimes a civil divorce is meant to protect a spouse in the case of abuse.
When the gift of sex is viewed selfishly as it is pretty much so today, then, people begin to see nothing wrong with the above sins. Living a marriage in Christ is truly living authentic love. This love is selfless. This love is life-giving.