From the Heritage Foundation:
The basic unit of society is the family and the cornerstone of the family is marriage, the union of one man and one woman. Deeply rooted in all societies, marriage is a fundamental social institution that has been tested and reaffirmed over thousands of years. The family yields significant “social capital” and other benefits to society, and children in an intact home have the most promising life prospects.
* Marriage is a fundamental social institution. Deeply rooted in all societies, marriage has been tested and reaffirmed over thousands of years. As an institution, marriage is the foundation of a harmonious and enriching family life and the basic building block of our society. It is not a casual relationship, but one that carries many obligations and benefits affecting husbands and wives, fathers and mothers, sons and daughters, and thus every individual in society.
* Marriage is the union of one man and one woman and must continue to be legally recognized as such. Marriage is not and has never been merely a private contract or personal consensual relationship, but a matter of common interest and public concern. It is a fundamental and universal institution whereby a man and a woman are joined together for the primary purpose of forming and maintaining a family. Individual marriages are recognized by the state, but the institution of marriage itself was not created—and thus cannot be redefined—by government.
* Children fare best and have the most promising life prospects when they are raised in intact families. Data from alternate household forms over the past several decades have reaffirmed that the intact family remains the best environment for children. The law has an interest in setting marriage apart from all other household forms. These others have not been able to guarantee the same level of security for the welfare of the next generation.
* A strong and broad consensus has emerged in the past decade that marriage must be strengthened. Policy and cultural efforts to reduce divorce and safeguard intact families have made progress. Our energy as a society should be directed toward strengthening this most basic social institution. Efforts to elevate the status of other household forms in the law detract from this movement to strengthen the unique institution of marriage.
* The institution of marriage has been shown to be overwhelmingly beneficial to children, adults, and society. For more than 50 years, government policy has discouraged marriage through the marriage penalties in the welfare system. Fortunately, there is now broad agreement that this pattern should be changed and that government should promote healthy marriage. It is now well understood that the family is the primary organization by which children are educated and formed into adults. As the cornerstone of the family, marriage produces children; provides children with mothers and fathers; and produces many social, economic, and health benefits for children, adults, and society generally. Promoting marriage has the potential to significantly decrease poverty and dependence, increase child well-being and adult happiness, and to provide the safest environment for women and children.