Rona,
Marriage is an institution. Wikipedia defines an institution as:
Institutions are structures and mechanisms of social order and cooperation governing the behavior of a set of individuals. Institutions are identified with a social purpose and permanence, transcending individual human lives and intentions, and with the making and enforcing of rules governing cooperative human behavior. The term, institution, is commonly applied to customs and behavior patterns important to a society, as well as to particular formal organizations of government and public service. As structures and mechanisms of social order among humans, institutions are one of the principal objects of study in the social sciences, including sociology, political science and economics. Institutions are a central concern for law, the formal regime for political rule-making and enforcement.
As an institution, marriage has what wikipedia calls "a social purpose which transcends individual lives and intentions". It is a mechanism of social order.
I know this sounds awfully cold and unfeeling. But cultural anthropologists study just such things. What are the values of a particular society? What are the rules? What are the institutions? How does the society govern or influence the behavior of its members? etc.
As wikipedia explains, the term institution is applied to customs and behavior patterns important to a society. Society uses institutions to promote social behavior deemed beneficial to the society as a whole. Our society (and, in fact, practially every society known to man) has determined that it is beneficial to the society as a whole to promote marriage as a permenant union between members of the opposite sex, for the purpose of forming a family unity and raising children.
What if you don't like that definition of marriage? Does the existance of childless couples, for example, prove that marriage has nothing to do with procreation? Well, go back and look at the part of the definition which says, "As structures and mechanisms of social order among humans, institutions are one of the principal objects of study in the social sciences, including sociology, political science and economics." It's not a matter of each person defining marriage as they will. As an social institution, marriage can be studied objectively, to determing what behaviors society is attempting to govern, what rules society is using to promote those behaviors, and what benefits society is gaining from the enforcement of said behaviors. You can even determine at what cost, such as at the cost of not allowing every single member of the society the ability to marry whomever they please. Which takes me back to the 4 restrictions to marriage in our society: you cannot marry (1) a minor, (2) a close relative, (3) a person who is already legal married, and (4) a person of the same sex.
So, what about divorce, single parents, and childless couples? Our society allows all three, but we do not promote them. We tell children, "One day, when you grow up and get married,...", "One day, when you are a daddy,...", "One day, when you have your own family..." We don't say, "When you grow up and get divorced..." Because what our society wants to promote is its members joining in permanent union with a member of the opposite sex, for the purpose of creating a family unit and raising children.
If our society changes the definition of marriage to include same-sex marriages, we will not be merely "accepting" same-sex unions, we will be promoting them. And institutions are very powerful forces for influencing behavior in a society.
