Here is an excellent explanation from Mercator Net explaining the importance of traditional marriage, and why marriage between a man and a woman is so special.
What is marriage?
Marriage is a natural institution where a man and a woman give themselves to each other exclusively for life in a sexual relationship that is open to procreation. It is publicly recognised, honoured and supported because of its unique capacity to generate new human life and to meet children’s deepest needs for the love and attachment of both their father and their mother. Marriage is different and distinguished from other sexual or caring relationships because of its permanence, its natural orientation to life, and the way it brings together and expresses the fullness of humanity in male and female.
Why does marriage have to be about the ability to have children? Older couples and infertile couples have always been allowed to marry.
When a married couple cannot have children, for reasons of age or infertility, they are still truly married because their lovemaking is designed to give life, even if it cannot give life at a particular point in time, or ever. Their sexual union is procreative by its nature, because husband and wife unite in an act that is naturally meant for the creation of a new human being. This is why sex deserves to be treated with a special reverence.
But surely marriage is more about two people in love than what kind of sex they have. Why is procreative sex special?
Sexual intercourse that is open to life is essential for marriage because marriage is not just a caring relationship between two people, but a union of love and life. In marriage a man and a woman pledge to love each other for life and to lovingly welcome and raise any children of their union.
Sadly, through the normalisation of casual sex, contraception, homosexual acts, condoms, abortion and IVF, our culture has denigrated and obscured the life-giving aspect of marriage and sexual intercourse. In spite of this, people still, deep down, know that the sexual act is about life – that it bonds a man and a woman together in a profound way because of the baby they may conceive.
People still sense the grandeur of the sexual act, its implicit promise of life-long love and commitment – “I will be here for you for always” – and this is why there is so much pain and heartache when sexual relationships break down or when a marriage is violated by adultery.
Isn’t the right to marry a basic human right?
“The right to marry and found a family” is written in the Universal Declaration of Human Rights (1948). But international human rights law has always understood and affirmed the enduring truth that marriage is a life-giving union of a man and a woman. TheUnited Nations Human Rights Committee, which monitors international human rights treaties, has stated that the right to marry “implies, in principle, the possibility to procreate”. The right to marry and found a family is a basic human right, but this right has an objective meaning and content – forming an open-to-life union with a person of the opposite sex.
But what about human dignity? Homosexual people can never feel that they are fully accepted and worthy of love if they are not allowed to marry their same-sex partner.
To love someone sexually means being able to accept them completely, including their fertility. Sexual acts that are closed to life, like anal sex, oral sex and contracepted heterosexual intercourse, may seem loving. But they cannot be truly loving because they reject the deepest part of the person’s sexuality – their capacity to give life, to be a father or a mother.
No one can deny that many homosexual persons sincerely care about their same-sex partners. But, as hard and painful as it is for those who suffer from same-sex attraction, real love demands chastity – the integration of sexual desires into unselfish love for the other person. This means abstaining from sex that is not marital and open to life.
Unfulfilled sexual desires can be a painful cross to carry. But a chaste life brings us true inner peace and joy, because we are living in harmony with the way our bodies have been designed and we are treating the person we love as a gift – loving him or her for their own sake, and not for the sexual pleasure they can give us.
From the Catechism:
1601 "The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament."
1602 Sacred Scripture begins with the creation of man and woman in the image and likeness of God and concludes with a vision of "the wedding-feast of the Lamb." Scripture speaks throughout of marriage and its "mystery," its institution and the meaning God has given it, its origin and its end, its various realizations throughout the history of salvation, the difficulties arising from sin and its renewal "in the Lord" in the New Covenant of Christ and the Church.
1603 "The intimate community of life and love which constitutes the married state has been established by the Creator and endowed by him with its own proper laws. . . . God himself is the author of marriage." The vocation to marriage is written in the very nature of man and woman as they came from the hand of the Creator. Marriage is not a purely human institution despite the many variations it may have undergone through the centuries in different cultures, social structures, and spiritual attitudes. These differences should not cause us to forget its common and permanent characteristics. Although the dignity of this institution is not transparent everywhere with the same clarity, some sense of the greatness of the matrimonial union exists in all cultures. "The well-being of the individual person and of both human and Christian society is closely bound up with the healthy state of conjugal and family life."
1604 God who created man out of love also calls him to love the fundamental and innate vocation of every human being. For man is created in the image and likeness of God who is himself love. Since God created him man and woman, their mutual love becomes an image of the absolute and unfailing love with which God loves man. It is good, very good, in the Creator's eyes. And this love which God blesses is intended to be fruitful and to be realized in the common work of watching over creation: "And God blessed them, and God said to them: 'Be fruitful and multiply, and fill the earth and subdue it.'"
1605 Holy Scripture affirms that man and woman were created for one another: "It is not good that the man should be alone." The woman, "flesh of his flesh," his equal, his nearest in all things, is given to him by God as a "helpmate"; she thus represents God from whom comes our help. "Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh." The Lord himself shows that this signifies an unbreakable union of their two lives by recalling what the plan of the Creator had been "in the beginning": "So they are no longer two, but one flesh."