In Politique et Société (“Politics and Society”), a recently published book-length interview with Pope Francis by Dominique Wolton, a French sociologist, Francis responds to a question about marriage: “Marriage can only be between a man and a woman. . . .We cannot change it. This is the nature of things, not just in the Church, but in human history.”
Francis thus undercuts the root objection to this assertion by those Catholics – such as Australian Jesuit Frank Brennan – who support the civil marriage of same-sex couples while distinguishing the latter from marriage in a sacramental sense. Canonist Edward N. Peters has already posted a devastating rebuttal  of this specious claim.
I have written on marriage in The Catholic Thing earlier, but here I would like to highlight – unpacking Francis’s response – the ontological basis of the truth of the Church’s teaching regarding marriage.
Is marriage a two-in-one-flesh union between a man and a woman because the Church says so, positing or postulating its existence and nature according to its own judgment, that is, Church law? If so, then, one accepts ecclesial “positivism,” the view that these are basically mere conventions. Indeed, Catholics, such as Johan Bonny, the Bishop of Antwerp, may be regarded as an ecclesial positivist because he gives as the only reason for rejecting same-sex marriage the fact that “Church law” says otherwise. (See my review  of Bishop Bonny’s book.) This positivism is similar to one thinking that human beings have rights because the state or society says so.
Alternatively, does the Church judge that marriage is a two-in-one-flesh union between a man and woman because that judgment is true to an objective reality, according to the order of creation? If so, then one is a Christian realist: marriage is grounded in the order of creation, of an independently existing reality, and therefore has an objective structure judged by the Church to be the case or the way things really are.
On this view, the fact that the Church teaches that marriage is a two-in-one-flesh union between a man and a woman adds nothing to the truth-status of this dogma. This realism about the truth-status of dogmatic propositions is similar to one holding that human beings have rights by virtue of their nature as human beings, and the state simply secures, rather than confers, those rights by writing them down in a constitution.
I contend that the Church holds to Christian realism. Vatican II’s Gaudium et Spes states, “The intimate partnership of married life and love has been established by the Creator and qualified by His laws.” (48) The Catechism of the Catholic Church adds, “The vocation to marriage is written in the very nature of man and woman as they came from the hand of the Creator” (no. 1603; 1614-1615).
The starting point of John Paul II’s theology of the body, too, is that sexual difference is grounded in an ontology of creation. In other words, the sexual difference between male and female is a creational given such that all mankind is bound to the structures of creation. It is also creational given that, at one and the same time, mankind is one and a bi-unity: male and female. John Paul explains:
Let us enter into the setting of the biblical “beginning.” In it the revealed truth concerning man as “the image and likeness” of God constitutes the immutable basis of all Christian anthropology. “God created man in his own image, in the image of God he created him; male and female he created them” (Gen 1: 27). This concise passage contains the fundamental anthropological truths: man is the high point of the whole order of creation in the visible world; the human race, which takes its origin from the calling into existence of man and woman, crowns the whole work of creation; both man and woman are human beings to an equal degree, both are created in God’s image.
Indeed, the pope imitates Christ (see Matt 19:3-9; Mark 10: 1-10) by appealing to the “beginning,” to the creational structure for marriage, drawing on Genesis 1 and 2 for his understanding of the normative intent of a biblical ontology of creation, the objective structures of creation, as they pertain to a bi-unity of husband and wife, united as complementary, bodily persons, in a two-in-one-flesh communion.
John Paul’s treatment of these foundational texts is ultimately theological, because it is grounded in an historical-redemptive dialectic of creation, fall (sin), redemption, and fulfillment . But it is also philosophical – it articulates a philosophical anthropology of the body-person, which in its broadest sense is man himself in the temporal form of existence of human life. Most significantly, the pope is arguing that, in the totality of the personal structure of man, his body is a basis, a substructure forming part of the unity of man and thus of the person. Indeed, the meaning of the human body is an integral part of the structure of the personal subject, rather than being “extrinsic to the person, the subject and the human act.”
The pope doesn’t deny that the inner structures and regularities of the human organism, per se, require scientific analysis and explanation. But he distinguishes the body as a “physiological unit” and the “bodiliness” of the human person. John Paul then argues, “the human body is not only the field of reactions of a sexual character, but [rather] it is at the same time the means of the expression of man as an integral whole, of the person, which reveals itself through the ‘language of the body’.”
“This ‘language’ has an important interpersonal meaning,” he adds, “especially in the area of the reciprocal relations between man and woman.” As I’ve argued here , this shows that the “‘language of the body’ should express, at a determinate level, the truth of the sacrament,” namely, a one-flesh union. “So then they are no longer two but one flesh.” (Mark 10:8)
It’s good that Pope Francis, too, seems to have affirmed these truths.