To understand what marriage is, the best place to start is with the human person. After all, marriage is a unique relationship between two specific persons, one man and one woman. We must ask, “What does it mean to be a human person, as a man or as a woman?” First, men and women are created in the image of God (see Gen 1:27). This means that they have great dignity and worth. Also, since “God is love,” (1 Jn 4:8) each person – created in God’s image – finds his or her fulfillment by loving others. Second, men and women are body-persons. The body – male or female – is an essential part of being human. Gender is not an afterthought or a mere social construct. The body shapes what it means to love as a human person. To sum up, when we think about marriage, we must think about who the human person is – created with great dignity, and called to love as a body-person, male or female.

“God himself is the author of marriage” (GS, no. 48). When God created human persons in his own image, as male and female, he placed in their hearts the desire, and the task, to love – to give themselves totally to another person. Marriage is one of two ways someone can make a total self-gift (the other is virginity, devoting oneself entirely to God) (see FC, no. 11). Marriage is not something thought up by human society or by any religion – rather, it springs from who the human person is, as male and female, and society and religion affirm and reinforce it. The truth of marriage is therefore accessible to everyone, regardless of their religious beliefs or lack thereof. Both faith and reason speak to the true meaning of marriage.

Marriage is the lifelong partnership of mutual and exclusive fidelity between a man and a woman ordered by its very nature to the good of the spouses and the procreation and education of children (see CCC, no. 1601; CIC, can. 1055.1; GS, no. 48). The bond of marriage is indissoluble – that is, it lasts “until death do us part.” At the heart of married love is the total gift of self that husband and wife freely offer to each other. Because of their sexual difference, husband and wife can truly become “one flesh” and can give to each other “the reality of children, who are a living reflection of their love” (FC, no. 14).

Marriage between a baptized man and a baptized woman is a sacrament. This means that the bond between husband and wife is a visible sign of the sacrificial love of Christ for his Church. As a sacrament, marriage gives spouses the grace they need to love each other generously, in imitation of Christ.

The word “marriage” isn’t simply a label that can be attached to different types of relationships. Instead, “marriage” reflects a deep reality – the reality of the unique, fruitful, lifelong union that is only possible between a man and a woman. Just as oxygen and hydrogen are essential to water, sexual difference is essential to marriage. The attempt to “redefine” marriage to include two persons of the same sex denies the reality of what marriage is. It is as impossible as trying to “redefine” water to include oxygen and nitrogen.

Sexual difference is the difference of man to woman and woman to man. It affects a person at every level of his or her existence: genetically, biologically, emotionally, psychologically, and socially. Sexual difference is an irreducible difference. It is unlike any other difference we experience, because it – and only it – allows for the total personal union between husband and wife that is at the heart of marriage. The difference between men and women is for the sake of their union with each other. It is what makes spousal union possible.

Of course love and commitment are important for marriage – as they are for many relationships. But marriage is unique because the commitment it calls for is better described as communion, where “the two become one flesh” (Gen 2:24). Only a man and a woman in marriage can become a “one flesh” communion. The unity of husband and wife is so intimate that from it can come a “third,” the child – a new life to be welcomed and raised in love. No other relationship, no matter how loving or committed, can have this unique form of commitment – communion – that exists in marriage, between a husband and a wife.

Gender matters for marriage because the body matters for love. My body is not simply “the shape of my skin.” Instead, my identity as a person (my “I”) is inseparable from the reality of my body – I am a body-person. As John Paul II said, the body reveals the person. It is a deeply personal reality, not just a biological fact (see TOB, sec. 9.4). The body is “taken up” into every human action, including the most important task of all: loving. Loving as a human person means loving as a man or as a woman. Marriage, the “primary form” of human love (GS, no. 12), necessarily involves the reality of men and women as body-persons. Marriage is intrinsically opposite-sex. To “write off” the body, and gender, as unimportant to marriage means treating the body as inconsequential or, at best, as an object or tool to be used according to one’s pleasure, instead of as an essential – and beautiful – aspect of being human and loving as a human person. Such a write-off would ignore the very essence of what marriage is.

The love between a husband and a wife involves a free, total, and faithful mutual gift of self that not only expresses love, but also opens the spouses to receive the gift of a child. No other human interaction on earth is like this. This is why sexual intimacy is reserved for married love – marriage is the only context wherein sex between a man and a woman can speak the true language of self-gift. On the other hand, sexual behavior between two men or two women can never arrive at the oneness experienced between husband and wife, nor can these acts be life-giving. In fact, it is impossible for two persons of the same sex to make a total gift of self to each other as a husband and a wife do, bodily and personally. For this reason, such sexual behavior is harmful and always wrong, as it is incapable of authentically expressing conjugal love – love which by its nature includes the capacity to give oneself fully to the other and to receive the other precisely as gift in a total communion of mind, body and spirit. Therefore, no relationship between two persons of the same sex can ever be held up as equal or analogous to the relationship between husband and wife.

“Complementarity” refers to the unique – and fruitful – relationship between men and women. Both men and women are created in the image of God. Both have great dignity and worth. But equality does not mean “sameness”: a man is not a woman, and a woman is not a man. Instead, “male and female are distinct bodily ways of being human, of being open to God and to one another” (LL, p. 10). Because men and women are “complementary,” they bring different gifts to a relationship. In marriage, the complementarity of husband and wife is expressed very clearly in the act of conjugal love, having children, and fathering and mothering –actions that call for the collaboration – and unique gifts – of husband and wife.

The Catholic Church cares about marriage because marriage is a fundamental good in itself and foundational to human existence and flourishing. Following the example of Jesus, the Church cares about the whole person, and all people. Marriage (or the lack thereof) affects everyone. Today, people all over the world are suffering because of the breakdown of the family – divorce, out-of-wedlock childbearing, and so on. Marriage is never just a “private” issue; it has public significance and public consequences. One only has to think of the connection between fatherless families and young men in jail to know that this is true. In addition, the proposal to “redefine” marriage to include two men or two women is really a proposal to “redefine” the human person, causing a forgetfulness of what it means to be a man or a woman. This is a basic injustice to men and women, children, and fathers and mothers. Marriage is truly one of the most important social justice issues of our time.

The videos in the Marriage: Unique for a Reason series artistically explain the essential aspects of marriage, as well as why upholding marriage contributes to the common good. Check out “Made for Each Other” and “Made for Life,” as well as their companion Viewers’ Guides and Resource Booklets. Also, please visit the Church Teaching page to read Church documents on marriage.