The Catholic Church, in Humanae Vitae and other documents, condemns contraception because the person who uses it to prevent pregnancy intentionally thwarts the primary end of the sexual act, procreation (the secondary and complementary end is unitive love). This condemnation is thus based on two related claims: 1) sex is ordered toward procreation as its natural end, and 2) morally good behavior involves behavior that at least doesn’t direct an act away from the natural end of that behavior, and thwarting that end—in the case of sexuality, the procreative end—renders the behavior immoral.
I’ve addressed both claims in previous articles: “Bringing Sanity Back to Sex Part I” and “Sex, For Goodness’ Sake.” Here I want to address a challenge that critics may pose to the idea that sex targets procreation as its natural goal. Such a challenge might go like this:
If sex were essentially about procreation then every act would result in conception because a thing cannot be without that which belongs to its essence. But not every act achieves conception due to infertility, old age, pregnancy, physical defect, or number of reasons. Therefore, it would seem that sex is not essentially about procreation. And if that is true, then respect for the procreative end of sex is not necessary for moral sexual behavior.
Does a particular instance of sex’s involuntary failure to achieve generation serve as a counterexample to the claim that sex is essentially a procreative type of activity?
A distinction must be made between particular instances of sex and the sexual act in itself. It’s not particular instances of sex that define its essence, but rather the general order that sex has to procreation given the kind of act it is. In other words, it is an act that is procreative in kind, naturally ordered toward its goal regardless of whether it achieves that goal or not.
A sexual act that does not achieve its goal of procreation is still a procreative act. In the language of the philosophers, the act retains its per se destinatus (its intrinsic destination). The act itself still has procreation as its destination; it is still the kind of act that is naturally ordained to achieve procreation.
Perhaps some examples will help illustrate the point. A baseball team is structured to win baseball games. Even if it fails to win games, this does not change its fundamental orientation and what its players are trying to do every time they play. What about an eye that can no longer see due to natural or accidental causes? Do such impediments mean the human eye by its nature is no longer for the sake of seeing?
One more: digestion involves many different parts—salivation, chewing, swallowing, and finally intestinal absorption of nutrients. Each of these parts has the nourishment of the organism as its destination or goal, even if the final goal of nutrient absorption is not achieved—whether in full or in part. In this case, the digestive parts would still retain their per se destinatus.
Similarly, even though the conjugal act may not achieve its final destination of conception the act still is “in itself” (per se) ordered toward that goal. The structure of the reproductive act remains the same because the sexual faculties are used in accord with nature’s design, and thus the act is natural. And as the late Michael Cronin concludes, since “the circumstances necessary for the due performance of our own share of the act are present, the act is lawful” (Science of Ethics Vol. I).
In his 2015 article, “A Realist Sexual Ethics,” philosopher Micah Newman suggests that another way to respond to this objection is to cast sex in terms of an “Aristotelian categorical,” which takes the form of S’s are F where S refers to a species and F refers to that which is said of a species. For example, the statement “Cats are four-legged animals” would be an Aristotelian categorical. It is a true description of the species that conveys a general norm. But just because a particular cat happens to not have four legs it doesn’t follow that the Aristotelian categorical concerning cats is proven false. Such a cat would still be a cat, but given the Aristotelian categorical it would be a defective instance of that general category.
To use Newman’s example, the statement “Humans have five fingers on each hand” is true of the species. But a person who happens to have six fingers on one of his hands is not thereby less human. Given the Aristotelian categorical, such a trait would be merely defective for a human being as opposed to it being a normal trait for an entirely new species.
A similar line of reasoning can be applied to sex considered universally, relative to its particular instances. Cast in terms of Aristotelian categoricals, the species of the sexual act is such that it is generative in kind—an act that by its nature aims at reproducing the human species. A particular instance of sex may, whatever the intent of the participants, fail to result in a pregnancy. If so, it is merely a defective instance of sex (naturally speaking, not morally) rather than categorically a different type of activity. The essence of the act remains the same: it is a reproductive type of activity.
Moral defect comes into play only when there is an intentional thwarting of sex’s natural order to procreation. Such action is morally defective because it intends from the beginning the lack of order to sex’s due end or goal. It treats the sexual act (a procreative act) as if it were something that it’s not (a non-procreative act).
It’s comparable to eating with the intention to vomit out the food afterward, willfully directing the act of eating (a body-nourishing act) away from its natural end of nourishing the body. Perversion is another word for this.
When you think it through, it becomes clear that merely because a particular instance of the sexual act doesn’t achieve its end of procreation, it doesn’t follow that sex is not for procreation. Since sex targets procreation as its natural end, we can use such order as a moral guide for evaluating appropriate sexual behavior.
 This formulation is taken from Edward Feser, Aquinas: A Beginner’s Guide, chapter 5.