It is truly a sad moment in human history that we are forced to debate what marriage is, but here we are. This paper from the Heritage Foundation titled "Marriage: What It Is, Why It Matters, and the Consequences of Redefining It" gives a thorough answer to the question, and I highly recommend that you read it in its entirety. Here is a quote from the paper:
Abstract
Marriage is based on the truth that men and women are complementary, the biological fact that reproduction depends on a man and a woman, and the reality that children need a mother and a father. Redefining marriage does not simply expand the existing understanding of marriage; it rejects these truths. Marriage is society’s least restrictive means of ensuring the well-being of children. By encouraging the norms of marriage—monogamy, sexual exclusivity, and permanence—the state strengthens civil society and reduces its own role. The future of this country depends on the future of marriage. The future of marriage depends on citizens understanding what it is and why it matters and demanding that government policies support, not undermine, true marriage.
Marriage is the fundamental building block of all human civilization. The government does not create marriage. Marriage is a natural institution that predates government. Society as a whole, not merely any given set of spouses, benefits from marriage. This is because marriage helps to channel procreative love into a stable institution that provides for the orderly bearing and rearing of the next generation.
This understanding of marriage as the union of man and woman is shared by the Jewish, christian, and Muslim traditions; by ancient Greek and roman thinkers untouched by these religions; and by various Enlightenment philosophers. It is affirmed by both common and civil law and by ancient Greek and roman law. Far from having been intended to exclude same-sex relationships, marriage as the union of husband and wife arose in many places, over several centuries, in which same-sex marriage was nowhere on the radar. Indeed, it arose in cultures that had no concept of sexual orientation and in some that fully accepted homoeroticism and even took it for granted.
As with other public policy issues, religious voices on marriage should be welcomed in the public square. Yet one need not appeal to distinctively religious arguments to understand why marriage—as a natural institution—is the union of man and woman [emphasis mine].
Abstract
Marriage is based on the truth that men and women are complementary, the biological fact that reproduction depends on a man and a woman, and the reality that children need a mother and a father. Redefining marriage does not simply expand the existing understanding of marriage; it rejects these truths. Marriage is society’s least restrictive means of ensuring the well-being of children. By encouraging the norms of marriage—monogamy, sexual exclusivity, and permanence—the state strengthens civil society and reduces its own role. The future of this country depends on the future of marriage. The future of marriage depends on citizens understanding what it is and why it matters and demanding that government policies support, not undermine, true marriage.
