Purpose and Definition of Marriage

Biologically it takes a man and a woman to create a child. The woman plays the primary role in that creation putting her health, strength, body, and even sometimes her life, on the line before and during the birth of the baby. Then after the baby is born the woman with her body continues to provide the best nourishment for the child for many months. She sacrifices a tremendous amount of time to provide that nourishment. Ideally the man will continue to be involved and provide the support making it possible for the woman to give her time and energy to the child. The child continues to remain very dependent on both parents for many years. Sacrifices must be made by both parents to meet the needs of that child.
While children can be very resilient and survive a lot of neglect physically, neglect by either or both parents does long term emotional damage.
Because the actual physical effort in creating the child is very minimal on the mans part, I'm sure it didn't take very long for the woman to require a commitment before she would consent to taking a chance in creating a child. That commitment is what is known as marriage. Whether the man had one or many wives, that commitment was still between the one man and the one woman. Since for most of the history of the world "productive people equaled power", governments backed up that commitment with laws and even special privileges. Thus we have the basic essence of marriage with its legal privileges.

And now we come to a point in our culture where we try to redefine marriage. Biologically it will not change anything but legally those privileges were meant for a specific purpose. Namely, the protection of the woman and child.
As medical advances and technology have changed our society, that primary purpose for legal marriage has become blurred. Abortion and birth control make it safe for a woman to indulge in what could have created a child without the risk or the need for commitment. Machinery and technology have taken over for and replaced people. Governments no longer need more people to have more power. Therefore governments no longer have that vested interest in the marriage commitment.
Children in our society have lost their value. They seem now only to be an indulgence and for many when they get too hard to care for they are in a sense thrown away.
But even with all those changes, for those who choose to value children, that primary commitment of marriage is vital. With all that the children have against them in our culture, those legal laws and privileges of the marriage commitment are probably even more important.
For the sake of the children, our society cannot afford a redefinition of marriage.