From “Persona Humana“, Section VII
Today there are many who vindicate the right to sexual union before marriage, at least in those cases where a firm intention to marry and an affection which is already in some way conjugal in the psychology of the subjects require this completion, which they judge to be connatural. This is especially the case when the celebration of the marriage is impeded by circumstances or when this intimate relationship seems necessary in order for love to be preserved.
This opinion is contrary to Christian doctrine, which states that every genital act must be within the framework of marriage. However firm the intention of those who practice such premature sexual relations may be, the fact remains that these relations cannot ensure, in sincerity and fidelity, the interpersonal relationship between a man and a woman, nor especially can they protect this relationship from whims and caprices. Now it is a stable union that Jesus willed, and He restored its original requirement, beginning with the sexual difference. “Have you not read that the Creator from the beginning made them male and female and that He said: This is why a man must leave father and mother, and cling to his wife, and the two become one body? They are no longer two, therefore, but one body. So then, what God has united, man must not divide.” St. Paul will be even more explicit when he shows that if unmarried people or widows cannot live chastely they have no other alternative than the stable union of marriage: “. . .it is better to marry than to be aflame with passion.” Through marriage, in fact, the love of married people is taken up into that love which Christ irrevocably has for the Church, while dissolute sexual union defiles the temple of the Holy Spirit which the Christian has become. Sexual union therefore is only legitimate if a definitive community of life has been established between the man and the woman.
This is what the Church has always understood and taught, and she finds a profound agreement with her doctrine in men’s reflection and in the lessons of history.
Experience teaches us that love must find its safeguard in the stability of marriage, if sexual intercourse is truly to respond to the requirements of its own finality and to those of human dignity. These requirements call for a conjugal contract sanctioned and guaranteed by society – a contract which establishes a state of life of capital importance both for the exclusive union of the man and the woman and for the good of their family and of the human community. Most often, in fact, premarital relations exclude the possibility of children. What is represented to be conjugal love is not able, as it absolutely should be, to develop into paternal and maternal love. Or, if it does happen to do so, this will be to the detriment of the children, who will be deprived of the stable environment in which they ought to develop in order to find in it the way and the means of their insertion into society as a whole.
The consent given by people who wish to be united in marriage must therefore be manifested externally and in a manner which makes it valid in the eyes of society. As far as the faithful are concerned, their consent to the setting up of a community of conjugal life must be expressed according to the laws of the Church. It is a consent which makes their marriage a Sacrament of Christ.