We are rational animals, and our “natural” (correct) activities are rationally chosen; we can reason whether to eat a meal or not, whether to eat everything in sight or not, etc.
Everyone knows that something like masturbation is not in accordance with reason (meaning that reason looks at the body’s construction, looks at the difference between male and female, looks at the result of copulation [pregnancy, birth, rearing of a child], and from reasoning about this reason concludes as natural law that the reasonable purpose of sex is for the continuation of the species, with pleasurable aspects of sex assisting humans in desiring to engage in the long-term requirements of its practice). Therefore, a rational person (one acting from reason rather than animalistically from appetite) knows a natural law that sexual activity is for species continuation.
And since we are a species living in community, we develop communal laws about how we as a society will use this activity so as to support the community growth rather than exploit one another, and we have human laws about sex.
And finally for the rational Christian, we have a revealed law from God that wants us to not only have rational outer activity but also have it in the heart, in the will, in the understanding; God’s Law.
Everyone knows it is a sin, that is why we do not drive down the street and see people paused along the sidewalk masturbating. Everyone who does it, does it in secret, hiding.
They are “turning away” from conscience, from reason, from natural law, and from God. I will wager that they are not saying the Our Father or any other real prayer while masturbating, and are pretending that God is not there with them, right in front of them, looking squarely at them as they do it. No, everyone knows it is a sin, not moral, and they pretend it never happened when they come out of hiding and again begin talking with others in their community.