Sex before marriage
Why does living together first count against us?
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.
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.
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. Cf. FC 81. 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.
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 — Congregation for the Doctrine of the Faith, 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.