Why does the Church teach that marriage is between a man and a woman?

To understand this teaching, it helps to see what the Church believes marriage is, rather than starting from what it excludes. In the Catholic view, marriage isn't a label society invented and is free to reassign; it is a reality God built into human life from the beginning. Scripture describes God creating humanity male and female and joining man and woman as "one flesh." The Church reads this not as an arbitrary cultural arrangement but as a revelation of what marriage actually is: the union of two persons whose very difference makes their joining a real complementarity, body and soul.

Two threads run through this. The first is complementarity — the conviction that man and woman are made to fit together, not interchangeably but as genuinely different and mutually completing. The second is openness to life. In God's design, the love of husband and wife is the same love from which children naturally come; the bodily union of a man and a woman is the one human relationship from which new life can spring. The Church holds these two — the uniting of the spouses and the openness to new life — together as inseparable to what marriage is. A union ordered toward both meanings is what she means by marriage, and only the union of a man and a woman is structured that way.

Jesus himself, asked about marriage, pointed back to "the beginning," to God's creation of male and female and the one-flesh bond, and treated that as the permanent truth of marriage rather than a custom to be revised. The Church understands herself to be bound by that, not free to redefine marriage even when a culture wishes she would. So when she says marriage is between a man and a woman, she is not making a fresh rule; she is professing what she believes marriage has always been.

It is vital to hear what this teaching does not mean. It is not a verdict on anyone's worth, and it carries no permission for contempt. The Church teaches with real seriousness that people who experience same-sex attraction are to be welcomed with respect, compassion, and sensitivity, and that every form of unjust discrimination against them is wrong. They are beloved children of God, called like everyone to holiness, friendship, and a life rich in love. The Church's teaching on the nature of marriage and her insistence on the dignity of every person are meant to be held together, never used as a weapon.

This is genuinely hard territory, and the Church knows it asks something costly of many people. She offers her teaching not as an easy answer or a slogan, but as what she believes to be true about love, the body, and God's design — held out with charity, and always alongside the call to treat one another as God treats us: with mercy and respect.

What we believe marriage is shapes how we understand love, the body, children, and the family. The Church's teaching here flows from her larger conviction that the body and its differences mean something — that male and female, and the union open to new life, are not interchangeable accidents but part of God's design. At the same time, how this teaching is spoken matters enormously. Held with charity, it defends both the meaning of marriage and the dignity of every person; spoken with contempt, it betrays the very Gospel it claims to serve.

• Genesis 1:27-28 — humanity created male and female and blessed to be fruitful.
• Genesis 2:24 — the "one flesh" union of man and woman.
• Matthew 19:4-6 — Jesus grounds marriage in God's creation of male and female "from the beginning."
• Mark 10:6-9 — a parallel account of Jesus pointing back to creation and the permanence of marriage.
• Ephesians 5:31-32 — the one-flesh union read as a sign of Christ and the Church.

• CCC 1601, 1602, 1603, 1604, 1605, 2357, 2358, 2359