2349 "People should cultivate (chastity) in the way that is suited to their state of life. Some profess virginity or consecrated celibacy which enables them to give themselves to God alone with an undivided heart in a remarkable manner. Others live in the way prescribed for all by the moral law, whether they are married or single." (Congregation for the Doctrine of the faith, Persona humana 11) Married people are called to live conjugal chastity; others practice chastity in continence:
There are three forms of the virtue of chastity: the first is that of spouses, the second that of widows, and the third that of virgins. We do not praise any one of them to the exclusion of the others. . . . This is what makes for the richness of the discipline of the Church.
The various forms of chastity.
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.
Offenses against chastity.
2353 Fornication is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children. Moreover, it is a grave scandal when there is corruption of the young.
If this isn't old enough, you can go back to:
1975 with the Congregation for the Doctrine of the Faith — Declaration on Certain Questions Concerning Sexual Ethics (Persona humana), Section XI, and
407 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. (CCC2350, 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. (YouCat 425)
To give your body to another person symbolizes the total gift of yourself to that person.
Pope John Paul II (1920-2005), meeting with young people in Kampala, Uganda, February 6, 1993
Young people want great things. . . . Christ did not promise an easy life. Those who desire comforts have dialed the wrong number. Rather, he shows us the way to great things, the good, toward an authentic human life.
Schönborn, Christoph, ed., Youcat English: Youth Catechism of the Catholic Church, trans. by Michael J. Miller (San Francisco: Ignatius Press, 2011), p. 222
You may not be aware, but it's only been fairly recently that society as a whole has accepted premarital sex and living together. Even into the 1980s, people who had sex outside of marriage were looked down upon by the majority in society; cultural acceptance of this practice generally started in the 1960s with the sexual revolution. Of course, sex outside of marriage has always gone on; I am referring to a society where living together or having sex outside of marriage is culturally accepted. So really, what's "new" is society accepting "live-in partners" and premarital sex. To be perfectly honest, the fact that premarital sex was morally wrong, was so obvious to people up until the 1960s that hardly anyone thought to assert it in writing or bring the topic up.
Yeah, people did it, but they did it knowing it was wrong and not caring. Now, people don't even know it's wrong.
For Catholics, the Church only recognizes weddings done within the church or by the permission of the bishop (exceptions are made for those who convert after getting married).
You say "As a daughter, I turned out well".
That is not the concern. The problem is that sex, outside of the life long public commitment of a husband and wife, incurs spiritual death to the partners, as the Sacred Scriptures testify because it is outside of God's design for human sexuality.
Jesus did come for the sinners, not for the righteous, and came as the Divine Physician to heal those who are ill, but when the ill person maintains that they are not ill (i.e., not sinning) and refuses to take the doctor's advice, it does them no good. The verse you are citing is,
5 31 Those who are well have no need of a physician, but those who are sick; 32 I have not come to call the righteous, but sinners to repentance.
He did not tell her it was OK to return to committing adultery (or fornication, for that matter).
Jesus came, not to affirm people in their sin, but to rescue them from it. When you are rescued, you acknowledge that you are in trouble, and stop doing what got you in trouble in the first place.
To maintain that it's OK to have sex outside of marriage (or, for Catholics, outside of a Church-sanctioned marriage) is not to respond to Jesus's call to repentance, but to refuse to be rescued from the damage that sin incurs (spiritual death).
It's hard to accept this in today's culture because it has so radically turned away from the Christian values it once upheld. Everyone is committing sexual immorality, and people expect it to happen. You're a freak if you don't participate. But Christians are called to be not of this world (cf. John 18:36) and to go against the current.
122 Do not be conformed to this world but be transformed by the renewal of your mind, that you may prove what is the will of God, what is good and acceptable and perfect.