How so?
I'll try and answer that one.
The reason is to protect the family. Historically and Biblically the family is the central institution in law and in society. Although we do not think of the family normally as a law-making body, the family is nonetheless the basic law-making body in all of history. Every point of power and authority is also a point of law, and historically, family law has been the basic law of mankind. In any society or institution, there are basic rules of conduct, and these rules of conduct constitute its law structure. The family is man's basic law-making body because of a variety of reasons, but certainly one of the first of these is the fact that its is the first place man, as a child, encounters law, rules of conduct, and his idea of law is shaped and defined to a great degree by the family. Life is seen through a law-structure which the family gives to the child, and this law structure defines life for the child. But this is not all. The child's attitude towards every other institution and its laws is largely shaped by the family. How the child approaches and reacts to church, school, state, and society depends greatly on his source of law, parental authority. He can face these other law making bodies rebelliously, or he can face them obediently. His attitude can be constructive, destructive, or indifferent, depending on his family background to a large degree. Every parent daily is a law-making person, a focal point of law-enforcement, and the delinquency of parents in this respect is the delinquency before God, their Lord and sovereign.
The family is man's first and basic school, and it is the first government in the life of a child. It is the father that is ordained by God as head of the household and his government is under God is the child's basic government. The child is not the only one governed by the family. The mother is governed by her activities by the requirements of her husband and children. The father is governed by the necessities of providing for the family, protecting them, and giving them the example and leadership they need. Where the family is not self-supporting, there is neither power nor authority in the person of the father. Welfare families, from the days of the Roman Empire to the present, have been notorious for their undisciplined, immoral, and delinquent characters, and welfare families have always been marked by a general lack of male authority. A man who will not provide for his family, accumalate respect, and cherish private property, will have neither the authority nor the ability to govern with wisdom and honor. Lacking self-government, he cannot govern others.
This brings me to another function of the family...it's economic function. The father provides for his family, not for strangers. Welfare agencies provide somewhat for millions of Americans, but the family system provides far more support for hundreds of MILLIONS! Under the family system, children are not only intellectually motivated for the best educational results, but they are economically financed through grade and high school, college, and sometimes graduate school. In terms of sheer economic effienciency, nothing in all of history has ever equalled the family. Compare this to the Communist failures. The family brings social stability and order to society.
This is one of the TRUE reasons for marriage. If the government chooses to redefine marriage, they will most certainly destroy the family and destroy our civilization.