Why is the Church against premarital sexual relations?
Because she would like to protect love. A person can give someone else no greater gift than himself . "I love you" means for both: "I want only you, I want all that you are, and I want to give myself to you forever!" Because that is so, we cannot, even with our bodies, really say "I love you" temporarily or on a trial basis. Many people take their premarital relationships seriously. And yet there are two reservations involved that are incompatible with love: the "exit option" and the fear of a child. Because love is so great, so sacred, and so unique, the Church teaches young people the obligation to wait until they are married before they start to have sexual relations.
How can you live as a young Christian if you are living in a premarital relationship or have already had premarital relations?
God loves us at every moment, in every "complicated" situation, even in a state of sin. God helps us to seek the whole truth about love and to find ways to live it more and more unambiguously and decisively. In a conversation with a priest or a reliable, experienced Christian, young people can look for a way to live out their love with increasing integrity. They will learn that every life is a process and that, whatever has happened, they can make a new beginning with God's help.
(From the Catholic Youth Catechism, questions 407-408)