Mk 2, 18-22
The disciples of John and of the Pharisees were accustomed to fast. People came to him and objected, "Why do the disciples of John and the disciples of the Pharisees fast, but your disciples do not fast?" Jesus answered them, "Can the wedding guests fast while the bridegroom is with them? As long as they have the bridegroom with them they cannot fast. But the days will come when the bridegroom is taken away from them, and then they will fast on that day. No one sews a piece of unshrunken cloth on an old cloak. If he does, its fullness pulls away, the new from the old, and the tear gets worse.
Likewise, no one pours new wine into old wineskins. Otherwise, the wine will burst the skins, and both the wine and the skins are ruined. Rather, new wine is poured into fresh wineskins."
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. (2350, 2391)
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.