Fornication is the sin committed by those who engage in pre-marital sexual intercourse, whether these are the brief encounters of promiscuous sex, the acts of those who have lived together for a long time, or the acts of the engaged who anticipate soon being marriage. Some acts of fornication are manifestly wrong since they are loveless acts that cruelly exploit another. Promiscuous acts do not express love for another; they are directly solely to sexual pleasure. Even if the exploitation is mutual, it is wrong, for one should never risk harming another, or allow one’s self to be harmed nor should one risk conceiving children one has no intent to care for.
Some who have sexual relations apart from marriage may avow committed love to one another, but in fact they have not made the commitments necessary to legitimate sexual involvement. In all instances, fornication is immoral since the individuals involved have not pledged themselves to the promises that are implicit in the sex act; they have made no binding pledge to be faithful to one another and to care for one another; they are endangering the well-being of any children they may conceive. Human relations are notoriously fragile and susceptible to dissolution for the weakest of motives. We need the bonds of marriage to help us be faithful to the promises implicit in sexual relations.
Many who are abandoned by their premarital lovers become less trusting and giving in future relationships, and may endanger their future marital relations. Sexual intercourse outside of marriage is also often accompanied by the sin of contraception, which in itself suggests the de facto temporariness of pre-marital relations.
Catechism of the Catholic Church 2353,2390-1
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.
2390 In a so-called free union, a man and a woman refuse to give juridical and public form to a liaison involving sexual intimacy.
The expression “free union” is fallacious: what can “union” mean when the partners make no commitment to one another, each exhibiting a lack of trust in the other, in himself, or in the future?
The expression covers a number of different situations: concubinage, rejection of marriage as such, or inability to make long-term commitments. All these situations offend against the dignity of marriage; they destroy the very idea of the family; they weaken the sense of fidelity. They are contrary to the moral law. The sexual act must take place exclusively within marriage. Outside of marriage it always constitutes a grave sin and excludes one from sacramental communion.
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.”
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.