Youcat commented through CCC – Question n. 407.

YOUCAT Question n. 407 - Why is the Church against premarital sexual relations?

(Youcat answer)  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.      

A deepening through CCC

(CCC 2350) Those who are engaged to marry are called to live chastity in continence. They should see in this time of testing a discovery of mutual respect, an apprenticeship in fidelity, and the hope of receiving one another from God. They should reserve for marriage the expressions of affection that belong to married love. They will help each other grow in chastity.      

Reflecting and meditating 

(Youcat comment) 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.

(CCC Comment)

(CCC 2391) Some today claim a "right to a trial marriage" where there is an intention of getting married later. However firm the purpose of those who engage in premature sexual relations may be, "the fact is that such liaisons can scarcely ensure mutual sincerity and fidelity in a relationship between a man and a woman, nor, especially, can they protect it from inconstancy of desires or whim" (CDF, Persona humana 7). Carnal union is morally legitimate only when a definitive community of life between a man and woman has been established. Human love does not tolerate "trial marriages." It demands a total and definitive gift of persons to one another) Cf. FC 80).

 
(The next question is: How can you live as a young Christian if you are living in a premarital relationship or have already had premarital relations?)