One of the best books on the argument for marriage the way that we understand it is "What is Marriage?: A Man and Woman: A Defense" by Ryan Anderson, Robert George, and Sherif Girgis. I'll review the book in a later post, but in the work, the authors systematically lay out the foundation for the conjugal view of marriage. Namely, that marriage isn't based on emotions, but rather on a mutual desire to express love in such a way that it flows out from the couple and into children. Along the course of the argument, the material touched on some auxiliary issues, including the intent required during sex. Wrongful intent in a sexual relationship, even wrongful intent held by a married person, can reduce sex to a lie or a tool.
Sex is an organic bodily union. The male and female reproductive systems carry out their own processes individually, but it's only through the combination of the two systems, through sex, that either is able to complete the reproductive cycle. While sex unites and binds the two people into one bodily system, it does more than simply complete a process. Sex unites both the body and the mind. Bodily systems do not think or act independently, they must be directed by the mind and by the heart. So when a couple has sex, their minds and hearts direct their reproductive systems to work towards a common good, namely, the creation of new life and the strengthening of the marital union. The fact that every sexual act doesn't achieve procreation is irrelevant. What is relevant is the intent of each spouse. The right intent is enough to achieve the good regardless of whether or not reproduction itself is achieved.
The intent of one spouse can denigrate or destroy the organic bodily union. In order to perfectly achieve the design of sex, both partners must be willing the good of the other, not simply attempting to achieve one's own maximum pleasure. When one spouse withholds emotionally or mentally, the act is diminished. The bodily systems function in the same way, but the wrongful intent reduces the sublime nature of the organic bodily union to simply checking off a box or completing a process. Even more egregious is that wrongful intent reduces the opposite spouse to an object, a means to achieve a singularly pleasurable end. Wrongful intent divides the heart and diverts precious resources away from the marital good and to a selfish objective.
Procreation as an end is good, but again, it's not required. There is a great deal of confusion about the Church's teaching which has driven scores of Catholics into the arms of the contraceptive culture. Think of it this way. If sex's only objective was the creation of new life, why would the female reproductive system only be able to achieve pregnancy a few days in a cycle? By looking to the natural order, unimpeded by contraceptives, we can discern that sex is about more than procreation. The natural order tells us that sex is also unitive.
Certainly removing the procreative aspect of sexuality is just as damaging as removing the unitive aspect. Sex and intent towards sex should respect both spouses, should be used responsibly in the creation of new life, and should always be prepared to lovingly and openly accept any children that may come about from the sexual act.
A responsible couple practicing the discipline of natural family planning is able to balance the unitive needs of the couple and the responsibility of parenting. Sex should be a regular occurrence in a healthy marriage because it binds spouses together in ways that nothing else can. At the same time, recognizing the grave responsibility of raising a child, natural family planning empowers a couple to make judicious and just decisions about when to attempt to achieve a pregnancy. By doing nothing more than observing and working with the wife's reproductive system, a couple is able to appropriately, morally, and intentionally seek to create new life when their family is able to support new life, and abstain from sex when they are not. A key foundational principle of natural family planning is the understanding that, despite the scientifically valid data proving natural family planning to be effective, like contraceptives, it's practice is susceptible to human error, meaning that pregnancy can still occur. The difference between contraceptives failing and natural family planning failing is in how the couple reacts. A contraceptive mindset leads to an intent that seeks to immediately destroy the life that was just created while a natural family planning mindset leads to an overwhelming joy and excitement about the child who in just a few months will be welcomed into the home.
The Church is the last remaining institution that celebrates and promotes true freedom. By helping us to understand the importance of the conjugal view of marriage and the right attitudes about sex, She encourages us to experience true freedom. We are not to be prisoners to our own minds and passions nor are we to be enslaved to the dire side effects of contraceptives. Instead, we are to experience the true beauty of the married life, and, if in accord with God's Will, the joys of parenting.