Whenever discussing points of ethical contention surrounding the complex subject of assisted reproductive technology, there is usually one fundamental question in need of answering in the church: What exactly is the one flesh union of marriage?

Thank you to Dr. Gifford A. Grobien, Assistant Professor of Systematic Theology and Director of the D. Min. Program at Concordia Theological Seminary, Fort Wayne, IN, for graciously thinking through this question and composing an answer for the benefit of our ongoing discussion of infertility medicine:

The instituting text for marriage is Genesis 2:24: “Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.” The English “flesh” translates well the Hebrew bāśār, meaning the material tissue of a creature. It is the organic, biological elements of a person. The Hebrew něp̄ěš refers to a complete creature, not just its flesh, but its flesh and life. For animals (e.g. Genesis 1:20) this includes its breath; for human persons this includes the soul or self. Něp̄ěš is not used in Genesis 2:23-25.

Thus marriage is characterized fundamentally not as a personal union, but as a fleshly union. This union is an organic, biological union of bodies. Marriage is, firstly, the becoming “one flesh,” not becoming one creature or person. Marriage is a union of flesh or bodies. Marriage is not identical to coitus, but coitus is constitutive of marriage.* When Jesus is questioned about divorce, he reminds his questioners about the nature of marriage as union of flesh, quoting Genesis 2:24 (Matthew 19:3-6; Mark 10:2-12). What, then, is this becoming one flesh? How is it different from becoming united in other ways?

Girgis, George, and Anderson (2010) explain that the union of flesh is not just bodily intimacy, in which body parts of one person surround or intermingle with the body parts of another person. Rather, a body serves natural life. The parts of a body coordinate to achieve biological purpose. For two people to have fleshly, or biological, union, “their bodies must be coordinated for some biological purpose of the whole” (p. 254). Nearly all biological, or fleshly, acts can be accomplished by one independent body—for example respiration, circulation, and digestion. Indeed, fleshly union with respect to any of these bodily acts is impossible. Only in coitus do two bodies act for one biological function.

Coitus enacts a unique fleshly existence and accompanying purpose of procreation. Alone, the reproductive system of one person cannot reproduce. Coitus brings together two bodies in a fleshly union to make possible the singular biological act of procreation.

Any bodily touching that is not coitus—even other touching of a sexual nature—is not true bodily union but only juxtaposition or contiguousness, even if this juxtaposition happens to occur inside a person’s body. One might argue that non-coital sexual relations nourish and expresses intimacy and emotional union. Yet such a union would be just that: one of emotion, the will, or the mind. It is still not a union of the flesh, by which two bodies act together as one body or one flesh, seeking a fleshly—that is, organic or biological—purpose.

The union of flesh in marriage, then, consists of coitus. To emphasize this does not, on the other hand, deny that marriage is also a union of minds, wills, and passions. Marriage by design includes all of these. Coitus is not the only element of marriage. Rather, coitus is one of the fundamental, unique elements of marriage.

Nor is emphasizing coitus to reduce marriage to mating, as though human persons were mere animals. Although, on the above criteria, many animals also engage in a union of flesh when they mate, this does not exclude other characteristics from the fleshly union of man and woman: characteristics which qualify human marriage differently from the mating of animals.

The fleshly union of man and woman is fundamentally a bodily union, but it also includes the union of other human qualities such as the will, the emotions, and the mind. Taking the above understanding of union in general to mean the coordination of two or more elements for a common purpose, in sexual relations a man and woman would also properly coordinate their wills, emotions, and minds. Indeed, their souls are coordinated and caught up with one another in the purposes of deepening and nourishing their relationship, of enjoying one another, and of conceiving, bearing, and raising a child. The relational bond is as much a part of the fleshly union as the biological union. To insist upon the biological or organic union as fundamental to marriage does not in any way marginalize the other ways that a husband and wife are united in marriage.

St. Paul explains this in Ephesians 5: “In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, because we are members of his body” (28-30). For human beings, fleshly union is more than mating; is it to treat one’s spouse with love, nourishment, and warmth or comfort. These are the human qualities that go along with fleshly union.

Nor does the fundamental character of fleshly union in marriage in any way diminish or annul the marriages of infertile couples. The union of flesh refers to the act of coitus. In coitus, man and woman come together as one organically. Should this act not later result in conception says nothing about the act of union itself. “[W]hether a couple achieves bodily union depends on facts about what is happening between their bodies,” not other factors regarding the effectiveness of the reproductive system (Girgis, George, and Anderson, 266).

Finally, it is, in fact, only through fleshly union that two people can be completely united. People of all sorts may be united emotionally, according to their wills, or according to their minds. Coworkers united to find the solution to a research question or to a mechanical problem in an automobile have a kind of union in intellect. Friends are united in common activities according to their wills and often according to their emotions. Bodily union, however, occurs only between two who engage in a union of the flesh. Thus, the only relationship that allows the full union of persons—bodily, emotionally, according to the will, and according to the mind—is the relationship which includes fleshly union, that is, marriage. Thus, again, St. Paul’s words in Ephesians 5 express the character of this union: a union of flesh, of love, of care, of growth and nourishment (28-30).

Marriage may be instituted as a union of flesh, a union of bāśār. As a union of human flesh, however, it rightly becomes a union of něp̄ěš, of life. The union of flesh, the ground of marriage, properly stimulates true love for one another, leading to a true union of lives, of both bodies and souls.

*In this essay, coitus means specifically male-female genital sexual relations, not any other kind of use of sexual organs, even that which may occur between a man and a woman.

Girgis, S., R.P. George, and R.T. Anderson. 2010. “What is Marriage?” Harvard Journal of Law & Public Policy. 34 (1): 245-288.