It is a big deal and not just to Christians.
Sex is not merely a physical thing. It is also connected the psychology and spirituality of a human being. For example, the natural purpose of sex is reproduction. When human beings reproduce, they also create emotional or psychological bonds, such as love, compassion, generosity, self-sacrifice, self-respect, etc. Normally, when people have a child, they are so proud of themselves and will do anything for that child.
Accordingly, when a woman bears child and miscarriages or when parents lose a child, they suffer sorrow, lamentation, pain, grief and despair. This happens because these emotional bondings are connected with the natural mechanisms of sexuality.
Through sexuality, human beings develop a sense of connectedness, integrity, belonging and self-fulfillment. "This person is my lover, this person is my best friend, my soul mate, my better half". Or "these people are my family, they are my obligation or duty, I must work for them and take care of them, etc".
Thus, psychological things like love, integrity and self-respect are part of the sexual mechanisms of human beings.
When one has sex before marriage, one plays with fire. If two people are absolutely certain they will marry eachother, then sex before marriage is OK but then, if they are certain then they would marry and therefore could wait until then to have sex. Why? This provides a good example to others and to their eventual children.
But most sex before marriage is just adventure, just indulgence and experimentation. From sex before marriage, human beings develop wrong views about sex such as it does not affect people psychologically. That it is just fun. That nobody yets hurt.
People develop addictions to sex, which is why when they get married they have difficulty maintaining it. Their lusts distract them. Instead of caring and working for children, they are out nightclubbing and children suffer from feeling rejected. Children develop pyschological problems.
Worst of all, sex before marriage leads to masses of confused, heartbroken people who have had their self-respect, self-esteem, self-love, integrity and sense of belonging shattered. For some people, it takes them years to get over their heartbreak because sexuality, as I stated, has a psychological component.
When someone falls in love with another person and the other person leaves them, it is the same as a parent losing a child. It is not different. All of these things come from our natural sexuality.
In brief, sex before marriage is playing with fire and a huge risk. Through it, many lose their natural capacity of how to relate to other people properly.
LASTLY, OF COURSE HUMAN BEINGS CAN STILL HEAL THEIR WOUNDS BY DEVELOPING HEALTHY VIEWS ABOUT SEX.