Young people, dealing with “hormones” can point to how "no one is hurt by “self gratification”.
So it seems that, in trying to explain to others why the church approves certain physical, as opposed to mental or emotional or spiritual, sexual activities and disapproves others has to do with the proper use of our physical bodies and this leads into an explanation of just why we even have sexual organs.
You’re right that the Church condemns masturbation because it violates the procreative aspect of marriage.
However it also violates the unitive purpose of sex. The teenager masturbating is not giving himself (or herself) to his spouse. He is not expressing or renewing wedding vows. As the Catechism states, the act of masturbation is not an act of self giving love to a spouse. Masturbation can be seen as sinful for this reason as well as a violation of the procreative act.
Now to do this I have attempted to get at the root core of what the sexual organs are designed for - and that is procreation. I can (and am) “unitive” with my wife without intercourse. In fact I would say that many people are “unitive” with their spouse before they are wed. The only thing that Changes in a Christian marriage after the wedding is the physical aspect. The emotional and spiritual one is already in place before hand.
I can be “unitive” without sex. I cannot (naturally) procreate without sex.
As I stated before, “unitive” doesn’t mean simply feelings of intimacy and closeness. That’s why it would be sinful for an unmarried or even an engaged couple to engaged in sexual intercourse, even without contraception. I’m sure other people can express it far better than I can.
Also, “the only thing that changes in a Christian marriage after the wedding is the physical aspect” is wrong. In the sacrament of matrimony, the Holy Spirit forms an unbreakable bond between the man and woman, making them an image of the union between Christ and His Church.
It is that unbreakable, total self giving, bond, that sex, the marital act, expresses and renews.
I read in one of your previous posts that your wife has Alzheimer’s disease. You have my sympathy, and I’ll pray for you.

No one is trying to diminish the admirable (indeed heroic:thumbsup

charity that you are showing your wife by discussing the unitive aspect of sex.
I’m not an expert on the Church’s teaching of human sexuality. I’ve never made any in depth study of the Church’s teaching, and I’m not married, so this is not something I’ve lived as a married woman. I’m sure there are other people out there who are experts (and who are married) who can explain it better than I can.
Anyway, this “root core” of something can be applied to anything. Someone mentioned a screwdriver earlier. An even better example might be a car. The Sole purpose and reason for a car to exist is transportation. If a car cannot fulfill this function it is called “broke down”. Every critical component on a car is designed to get you from here to there. Other things, like soft seats, heat and air conditioning, radio etc are all “add ons” and not critical to the “sole reason” of the existance of the car. Sleek design, high speed, and other “emotional” factors are also not necessary.
Similarly with the human sexual organs. They are designed for the purpose of procreation. Without that purpose, we would not have them at all.
Peace
James
No one is trying to deny the undeniable fact that sex is for procreation. This fact has been sadly forgotten our society, and you are right to point that out. But the Church teaches that they are not only for that purpose. Indeed, we might rightly ask, if human sexuality is no different than animals, why are human women able to have orgasms, unlike most other aimals? It serves no procreative purpose.
The Church does teach that the sexual act is for procreation. You are right about that! No one is denying that, in fact, that message is sorely needed in this world. However, the Church does not teach that the sole purpose of procreation. That is where your original post was incorrect.
I’m sure other people who have studied the Church’s teachings on human sexuality, especially those who have read Pope John Paul II’s Theology of the Body, could explain it far better than I can.

I turn the floor over to them.