WHY IS FORNICATION A SIN
Fornication was forbidden by God not due to a capricious decision of his, or due to a morbid desire to see us battle a natural impulse He made us with; but out of love and concern for us.
Humans were created with the capacity to feel sexual desire to facilitate intimacy and procreation. Without this, it would have been impossible for a man and a woman to get close to each other physically and emotionally, impossible to procreate and form families, and therefore impossible for civilization and progress to exist.
In its original state, sexual desire was under the control of the will. Even though it could have been aroused by the opposite sex, the faculty to accept or reject intercourse was perfectly under human control.
After the Fall, humanity lost control of its faculties, and became enslaved to them; being dragged from place to place by its own impulses.
(Titus 3:3)(Romans 6:12, 17-18)
WHAT HAPPENS IN FORNICATION
When fornication occurs, the powers of procreation and intimacy are released, provoking unwanted pregnancies and emotional cohesion between two people who are not meant or not ready to be together in a committed, blessed union before God.
Even in cases where pregnancy is avoided by the use of chemicals and other artefacts; which in itself is wrong, the sinfulness of this is manifested in abnormal pregnancies, cervical cancer, hormonal imbalance and other similar problems that would eventually damage future fertilization in a legitimate marriage.
Emotional ties are created, unwillingly, as direct result of intimacy. When the relationship brakes, the pain and trauma scar the psyche of the abandoned party making it more difficult to develop a normal relationship, while the damage will go on deeper and deeper the more it happens.
Finally, modesty is also demolished. Virginity as a sign of moral purity is taken away, making the fornicators more insensitive to further sin, by dulling their consciences.
Sexual union between a man and a woman who are not married were forbidden by God because it destroys the soul of those who practice it; destroys the psychological balance to create a formal family unit in the future, and corrupts the moral values and conscience of the individuals and the societies that practice it. (1Corinthians 6:9)
God never forbids something out of capriciousness. There is always a coherent, practical reason for it every time.