The reason comes from the fact that we are made in God’s image. As a result, we are created, like every other created thing, to respond to God - and human beings respond to God by being able to love God with “heart and soul and mind and might”. And the thing about human beings, is that we are free to do so, and are aware of doing so. We are created by God so that we might be united to Him by love, a love that He pours into our hearts, so that we can freely, willingly, love Him with it; for we cannot do so by our own strength.
As humans, we have bodies - and what we do with them, is intended to be an expression of love for Him. In loving us, God gives Himself totally and unconditionally to us - and wants us to be totally and unconditionally self-giving in return - in fact, commands it. That’s why obedience to Him, if perfect, refuses nothing to God.
And that is what Jesus has done - loved unconditionally & obeyed unreservedly. He has done so, and has made us parts of His own Body, so that we might be joined with Him, and our fellow-men, in being able to love God as we are meant to. He has taken us up into His own Life - that is what the life of grace is: it is the life of the Blessed Trinity; nothing less. God is so essential to us, that He cannot be replaced by anything created - we are created with a hunger and thirst for God, which cannot be satisfied except by God. As we cannot yet see God face to face, we practice loving God by loving our neighbour "“in God”; and married love is meant as an expression of this.
And as we are sexually differentiated, our use of sexuality is meant to be part of our total self-giving to God; it is a result of our being, not unembodied souls, not mere lumps of flesh, but a union of soul with body - a result of being created as human.
Sexuality is a problem because we are not as we are meant to be - human nature was created good and holy, but was spoilt by the Fall; so when we try to live on our own terms, and not on God’s, we’re trying to do what is impossible. Happiness is possible only in relation to God; for that is how we and all things are made. And our fallenness has spoilt our bodily nature - sexuality included. As Jesus Christ is our Redeemer, our sexuality is part of what is being redeemed. Which is a reason to act as the sort of people who are being redeemed.
All of us are called by Christ to carry the Cross - and disciplining our desires and bodies, and our sexual activity, is one way we do this, by His help. We cannot in this life escape the Cross - Jesus did so only by dying on it. What we can do, by being in union with Him, is offer to Him our difficulties in this, and every other, area of our lives. All our problems, including sexual ones, are reasons for us to seek His help; they are never reasons to keep away from Him. Our failures and weakness and our sins should never make us despair - when we do fall, we should rise.
Sex outside marriage is not wrong because the Church says so - the Church says so, because it is wrong. The Church is witnessing to the nature of marriage in saying this; because the total self-giving which married love ought to involve, is spoilt if any of that giving takes placeoutside marriage. The reason for this is that marriage is a miniature version of the Love of Christ for His Church - He gave Himself for her, without any reservation, & does still; and He will never divorce her - because He is one with her. Married love should be a total self-giving of each spouse to the other. And if some of that giving is not within marriage, the self-giving is less than it could have been. Which is a shame.
IOW, chaste sexual activity within marriage alone is a way of living as Christ’s disciples in the world. That’s why Christian families are called, and are, “domestic churches”. That is how we are meant to live - not for ourselves, because “your bodies are not your own; you are Christ’s, and Christ is God’s”. Our bodies don’t belong to us at all

- they belong to Christ. That is the basic reason for why the human body deserves respect - it is meant to be entirely His. To use our bodies for our own pleasure, without submitting them to God, is not fair to God, or to our neighbour - including the neighbour we intend to marry.
That’s why we have sacraments - to transform our soul and bodies, so that they can become fully Christ-like.
I hope that helps -
this might ##