From Pope St. John Paul II’s Apostolic Exhortation Familiaris Consortio (1981):
In its solicitude to protect the family in all its dimensions, not only the religious one, the Synod of Bishops did not fail to take into careful consideration certain situations which are irregular in a religious sense and often in the civil sense too. Such situations, as a result of today’s rapid cultural changes, are unfortunately becoming widespread also among Catholics with no little damage to the very institution of the family and to society, of which the family constitutes the basic cell.
A first example of an irregular situation is provided by what are called “trial marriages,” which many people today would like to justify by attributing a certain value to them. But human reason leads one to see that they are unacceptable, by showing the unconvincing nature of carrying out an “experiment” with human beings, whose dignity demands that they should be always and solely the term of a self-giving love without limitations of time or of any other circumstance.
The Church, for her part, cannot admit such a kind of union, for further and original reasons which derive from faith. For, in the first place, the gift of the body in the sexual relationship is a real symbol of the giving of the whole person: such a giving, moreover, in the present state of things cannot take place with full truth without the concourse of the love of charity, given by Christ. In the second place, marriage between two baptized persons is a real symbol of the union of Christ and the Church, which is not a temporary or “trial” union but one which is eternally faithful. Therefore between two baptized persons there can exist only an indissoluble marriage.
Such a situation cannot usually be overcome unless the human person, from childhood, with the help of Christ’s grace and without fear, has been trained to dominate concupiscence from the beginning and to establish relationships of genuine love with other people. This cannot be secured without a true education in genuine love and in the right use of sexuality, such as to introduce the human person in every aspect, and therefore the bodily aspect too, into the fullness of the mystery of Christ.
It will be very useful to investigate the causes of this phenomenon, including its psychological and sociological aspect, in order to find the proper remedy.
— Familiaris Consortio 79-80