CCC 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.¹ 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.

1 Cf. FC 81.