The proper end of the sexual urge is the existence of the species Homo, its continuation (procretio), and love between persons, between man and woman, is shaped, channeled one might say, by that purpose and formed from the material it provides. It can therefore take its correct shape only in so far as it develops in close harmony with the proper purpose of the sexual urge. An outright conflict with that purpose will also perturb and undermine love between persons. (52-3)
Wow! JPII is particularly referring to contraceptive acts here. A husband and wife must recognize the life-giving purpose behind the use of the sexual urge, and they must never do anything to thwart that purpose (i.e. sterilization, coitus interruptus, or use of barriers or chemical contraceptives of any kind). JPII says that "man often accords the sexual urge a merely biological significance and does not fully realize its true, existential significance - its link with existence" (53). When we think of something as merely biological, we tend to think it is okay to manipulate and alter it. Recognizing the "natural purpose of the sexual urge" as pro-creative and existential does not banish love from the equation. JPII says rather that recognizing the procreative purpose of the sexual urge gives conjugal love its true character (53).
In taking marital vows a man and a woman should be consciously choosing to "participate in the whole natural order of existence," being completely open to facilitating "the existence of another concrete person, their own child, blood of their blood, and flesh of their flesh" (53). As JPII says, "This person is at once an affirmation and a continuation of their own love. The natural order of human existence is not in conflict with love between persons but in strict harmony with it" (53).
Love between a man and woman is intimately connected with the utilization of the sexual urge. JPII explains that whenever a couple seeks to circumvent the purpose of the sexual urge by artificial means they are actually damaging the love between them.
There are of course times in which the sexual act simply cannot bring about the conception of a new human person (i.e. when a woman is not fertile - which is the majority of the time). Also, when a woman is already pregnant, if either of the spouses are sterile, or if a woman is post-menopausal, then conception is not biologically possible. JPII tells us that another value of the sexual urge, apart from its procreative purpose, is to bring together men and women into reciprocal and complementary relationships.
If the spouses are to take legitimate advantage of the energy which the urge releases, and of its natural promptings, they must before all else take into account its basic meaning and rationale. If this condition (that nothing shall be done to negate the proper purpose of the urge) is fulfilled, then even when a new human being cannot be born from the union of a man and woman, or from a particular occasion of sexual intercourse, the spouses are none the less reborn in love, and so to speak give birth to each other in their interpersonal communion. (f.n. 17, p294)
I am reminded of a passage from C.S. Lewis's Mere Christianity. I do not have the book, so I cannot quote it directly, but he speaks of the natural law and of morality regarding contracepted use of the sexual urge. He compares the use of the sexual urge to the use of the urge to eat. The primary purpose for hunger and the desire to eat is that we will indeed eat and be nourished and continue to live. (Remember, existence is our first and primary good :-)
However, there is pleasure attached to eating (unless of course the food tastes bad, is poisonous, or if you simply have no sense of taste or smell). This is comparable to the fact that pleasure is often attached to the use of the sexual urge, but pleasure is not guaranteed and is not a purpose of the sexual urge. By the way, Janet E. Smith in her talk "Contraception: Why Not?" makes the point that God attaches pleasure to those things which He wants us to do for our own continuation and health. Such things include sex, eating, sleeping, and exercising. I think that's fascinating.
Lewis compares contracepted intercourse to bulimia. The bulimic wants the pleasure of eating, but because he does not want to follow the act through to its natural end and experience its natural consequences, he forces himself to throw-up after he experiences enough of the pleasure of beginning the eating process. Likewise, contraception is an unnatural act by which a couple seeks to enjoy some of the pleasure of the sexual act without following it through to its natural end and experiencing its natural consequences. The consequence that most couples are trying to avoid when contracepting is the conception of a child. (The couple does not realize that there is much pleasure also to be had in the begetting of and rearing of their own children.)
Some couples have grave reasons for seeking to avoid pregnancy, particularly if the mother has a condition in which a pregnancy can cause her to die. The Church holds forth any number of modern, scientific Natural Family Planning methods for use in situations like these. (We are not talking "rhythm method" okay?)
Natural Family Planning (NFP) involves the charting of the wife's fertility-cycle. During the wife's "fertile window" the spouses mutually abstinence from intercourse (including any other act which would cause one of the spouses to come to sexual climax apart from the act of intercourse; that's a whole 'nother element of Catholic sexual moral teaching). The difference between NFP and contraceptive acts is that NFP refuses to utilize the sexual urge for the sole purpose of pleasure while intentionally thwarting the procreative capability of that act.
NFP also has some benefits in that it fosters communication between the spouses, prayer, a greater understanding of the woman's body and health, and also the virtues of self-control and self-discipline. During the woman's fertile period, the couple must seek creative, non-sexual ways to express their love. NFP can also help a couple to know when to "try for" a baby because they are aware of their fertile periods. It is still possible for a couple of use NFP with a "contraceptive mentality", but I do think the practice as a whole has a positive influence on the couple's openness to life and selflessness with one another.