It might be asked how a law can be contrary to the common good if it does not impose any particular kind of behavior, but simply gives de facto reality which does not seem to cause injustice to anyone. In this area, one needs first to reflect on the difference between homosexual behavior as a private phenomenon and the same behavior as a relationship in society, foreseen and approved by the law, to the point where it becomes one of the institutions in the legal structure. This second phenomenon is not only more serious, but also assumes a more wide-reaching and profound influence, and would result in changes to the entire organization of society, contrary to the common good. Civil laws are structuring principles of man’s life in society, for good or for ill. They “play a very important and sometimes decisive role in influencing patterns of thought and behavior”. Lifestyles and the underlying presuppositions these express not only externally shape the life of society, but also tend to modify the younger generation’s perception and evaluation of forms of behavior. Legal recognition of homosexual unions would obscure certain basic moral values and cause a devaluation of the institution of marriage.
Homosexual unions are totally lacking in the biological and anthropological elements of marriage and family which would be the basis, on the level of reason, for granting them legal recognition. Such unions are not able to contribute in a proper way to the procreation and survival of the human race. The possibility of using recently discovered methods of artificial reproduction, beyond involving a grave lack of respect for human dignity, does nothing to alter this inadequacy. — from Considerations Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Persons (1986)