God wants us to be happy and fulfilled, which is precisely why He designed things the way they are.
Catholics believe that sexuality has a fundamental purpose, decreed by God: procreation. Many Christians — who are fully willing to abide by what the Bible teaches — do not understand why the Bible teaches what it does about sex, even if they accept that it teaches certain things that have been normative in Christian cultures, to more-or-less degrees. They have no clue why certain things are prohibited, and other things required.
Moreover, having never been taught, many do not even dimly understand the distinctive Catholic teachings on sex, such as the prohibition of contraception.
Likewise, secularists and atheists and agnostics who ultimately don’t care what the Bible teaches, because they deny that it is revelation, and believe various myths about its nature and origins, want to hear non-biblical, non-religious secular, purely logical rationales for why we believe that certain sexual activities are wrong. This is my task as an apologist.
The Catholic Church teaches that it is a grave sin to deliberately separate sexuality from procreation, because the latter is its most essential purpose.
God created sex for this purpose, and for unity. Within its proper sphere, marriage between a man and a woman, he also established that spouses should experience pleasure and enjoyment. He created it for the deep fulfillment of human beings. Whatever is prohibited by Him is meant to foster this fulfillment, not to make people miserable and “incomplete”, etc.
We believe that when people follow the design that God has for sexuality, that they are the happiest, and that families and society prosper and flourish as a result (and indeed, this is sociologically demonstrable). To the extent that they do not follow the guidelines, the opposite will be the result.
Catholic sexuality is not anti-woman, anti-pleasure, anti-homosexual (persons), anti-natural desire. That’s how it's too often construed, because its nature or rationale isn't properly comprehended. God isn't against sex. He invented it. He described our relationship to Him as like a marriage.
In minimally graphic language, the Catholic view is that complete sexual fulfillment (including pleasure, which is not forbidden!) must occur in the act of love with one’s spouse of the opposite sex: that one is committed to for life, and that the couple must be open to life and possible conception. Sexual acts engaged in apart from this circumstance are wrong and sinful.
Contraception (deliberately thwarting a possible conception and engaging in sexuality under those circumstances) is wrong because it has an essentially “contralife will”: it insists on separating what ought not be separated (sexuality from possible conception, or being “open” to conception).
The Church teaches that a couple can space births and decide to postpone children or have no more children, for appropriately serious reasons of health, emotional factors and finances. This is what Natural Family Planning is about. The difference is that the practicing Catholic abstains from sexuality during the woman’s fertile periods, if they have legitimate reasons not to conceive a child.
Nor does the Church teach that every couple must have a dozen children. It acknowledges that a couple can plan sensibly regarding number of children and when to have them. But it does insist on sufficiently serious reasons for not having more children.
Blessed Pope Paul VI, in his landmark 1968 encyclical, Humanae Vitae, predicted several dire consequences for society and individuals, should contraception be widely practiced. They have all come true. Ideas and behaviors indeed have consequences. He could see the bad things on the horizon because he understood why contraception was wrong in the first place, and hence, would produce terrible fruits. Now we are living with those.
I'd like to suggest one way that we can defend this viewpoint from natural law, in an entirely non-religious, non-biblical way. We can better understand the Catholic prohibition of all forms of non-procreative sexuality by making an analogy to other organs and functions of the human body. Human beings instinctively believe that certain things are unnatural and shouldn't be separated.
My favorite example of this is taste buds and nutrition, in conjunction with eating. The “normal” understanding is that food should be enjoyed for its taste and also utilized for nutritional / health purposes. Both are, or should be present. We prove that this is what we believe, without thinking much about it, by our reactions to those who violate it.
So, for example, if a person completely separated the pleasure of taste from eating and insisted on eating bark, insects, rotten food (that still held nutritional value), we would consider that exceedingly strange and odd (as we do, the stereotypes of “sexually repressed” Puritans and Victorians).
Why? Well, it’s because we believe that food ought to be enjoyed while nourishing us. Taste buds have no relation to nutrition whatsoever. They are purely for sensory pleasure, and everyone believes that the pleasure shouldn't be separated from the nutritional aspect of food.
On the other extreme (the analogy to contraception), we have the junk food junkie. We think a person who exclusively eats Twinkies, chocolate-covered cherries, and cotton candy, or suchlike, is quite bizarre and not even remotely responsible about his or her diet. And that is because we know that food must have nutritional value, which is, in fact, its fundamental purpose, beyond merely enjoying its taste. Both have to be together.
This teaching does not mean that a married couple cannot engage in sexual activities when conception is not possible (such as the infertile periods for a woman or after her menopause). What is prohibited is a deliberate thwarting of a possible conception during fertile periods.
Many Catholics have never had the above reasoning and rationales explained to them. This often leads, sadly, to highly caricatured, stereotypical perceptions of Catholic teaching, including silly allusions to repressed nuns and dictatorial priests (and by extension, God Himself), who allegedly want everyone else to be as miserable as they supposedly are.
Satan's lies! God wants us to be happy and fulfilled, which is precisely why He designed things the way they are.