By observing our nature, we should come to realize that we are made for certain purposes. We see that all our faculties and associated abilities are directed toward an end, even if that end is never realized due to deformity (e.g. genetic, physical, mental abnormalities). For instance, if our digestive systems stop functioning correctly due to disease, we have to make do with the parts that function correctly. However, if we were to maliciously damage this system, we would be intentionally frustrating the ends toward which these faculties are intended. Similarly, our sexual organs, and, thus, sex itself, are directed toward particular ends (union and procreation), even if these ends are never realized due to defect. Sperm is always ordered toward fertilizing a woman’s ovum. The ovum is always ordered toward the reception of sperm. The sexual union of man and woman is the natural means of achieving the purpose for which our reproductive system, a system that is only complete when a man and woman unite sexually, is made.
Since human sexual intercourse is aimed at procreation, and the gifts of procreation, children, need a mother (nurturer) and father (protector) to continue the work they began in sex, then a permanent, committed relationship between a potential mother and father is necessary. “A parent of each sex is necessary to raise the child, because the male is better suited to protection and the female is better suited to nurture. A parent of each sex is necessary to teach the child, because the child needs a model of his own sex, a model of the other sex and a model of the relationship between them. Mom and dad are jointly irreplaceable.” Further, sexual pleasure is good, but it is not the end of the sexual act. It merely provides motivation for the sexual act, which must always be directed toward its proper ends.
Conversely, premarital sex is a seeking of gratification outside of a permanent, committed relationship. Additionally, it typically attempts to disrupt the end of sexual intercourse through some means of contraception. Even if contraception (to include withdrawal) is not used, the committed, permanent relationship needed to raise a child would not be present. “…fornication threatens to bring children into the world outside of the marital context they need for their well-being.” One can quickly see that this type of sexual union is devoid of moral union, because it seeks gratification without commitment. “[M]asturbation and pornography are also contrary to this inherently other-directed unitive drive insofar as they turn it inward toward a fantasy world rather than outward toward another human being.” Pornography also puts the marital act on public display, thereby, destroying the sacred nature God gave it.
Regarding chastity, since every human being has only half of a reproductive system, we can deduce that sex is not absolutely necessary. In other words, one does not have to use his/her sexual faculties in order to function or live. If one desired to set aside marriage and, therefore, sex, in order to pursue another noble endeavor, that person would be free to do so.
For more on these sins, read the following Catechism paragraphs: Marriage – 1602-1558; Chastity – 2337-2356; Masturbation – 2352; Pornography – 2354; Fornication – 2353.
 Dr. J. Budziszewski, The Meaning of Sexual Powers, 6.
 Edward Feser, “In Defense of the Perverted Faculty Argument,” _Neo-scholastic Essays_, 396.
 Feser, “In Defense of the Perverted Faculty Argument,” 396-97.