Excerpted from the Catechism of the Catholic Church.

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.[182] 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."[183] 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.[184]