Should the state (government) be concerned with the institution of marriage? This question is currently generating a lot of heat, but very little light. To answer this question with any certainty, we must first understand the purpose of government. It would seem that governments exist to preserve and protect. Who or what is preserved and protected depends upon the form of government and the ideal upon which it is founded. The state of a monarchy, founded upon the principle of the "divine right of kings", exists to preserve the lineage of the monarchy and protect the interests of the royal family. The state of a dictatorship, based upon the concept of "might makes right", is interested in preserving order and protecting the dictator and his colleagues. A communist state, which is founded upon the principle of "from each according to his ability; to each according to his need" [Karl Marx] tends, in practice, to reward the party faithful (the needy) and take from the general population (the able).
The American system of government, a democratic republic, is founded upon the principle of protecting the "unalienable rights" that have been endowed upon the people "by their Creator", "that among these are life, liberty, and the pursuit of happiness." Another concept that guides the government is the principle of protecting the interest of "the least of these." These are Christian principles. Thus, America is a Christian nation, not by religion, or the practice of religion, but by the underlying principles of Christianity that guided the founding fathers in their establishment of government.
There is significant historic evidence that the government has passed laws to protect the poor, the weak, and the disenfranchised. There are laws to protect certain minorities, women, and children. One example is E.E.O.C., which protects the employment interests of minorities and women. Many laws are passed to protect the interest of children, such as the child labor laws.
Does the state, then, have an interest in laws regarding marriage? The law regarding marriage has, as its primary focus, the products of marriage. What are the products of marriage? There are two - children and wealth. When a marriage breaks down and divorce is imminent, since the state has recognized and recorded the marriage, the state now has an interest. That interest is, again, to protect the interests of the weak and disenfranchised, namely the mother and children. The two issues to be resolved in any divorce court is child custody, including visitation and support, and the division of marital property. The interest of the state is to make sure that the children are properly supported and do not become wards of the state. By the same token, the division of the marital assets assures that the mother is not left destitute, and also becomes a ward of the state.
There are some underlying assumptions behind this reasoning that may be considered "old fashioned" or out dated, but nevertheless are part of the legal code. One assumption is that the husband, the man, is the head of the household and is the primary breadwinner. The wife, on the other hand, is the weaker party in this arrangement and requires protection under the law. Children are also considered the weaker parties and may even have their own advocate.
Given this description of the state's interest in marriage, and divorce, what interest does the state have in a same gender arrangement? It would seem that it has none. In a same gender relationship there is no product from the relationship in the form of children. It is biologically impossible. Any material gain produced by the relationship is already covered by laws regarding partnerships, inheritance, gifts, power-of-attorney, etc.
Notice that the state has no interest in whether this is a "loving relationship" or any other kind of relationship. It is not the state's responsibility to assess the nature of the relationship. Since most states have "no fault divorce laws", there is no reason for the state to even be interested in the nature of the relationship.
The religious aspects of marriage are addressed by the various religious organizations, who may set their own rules concerning who can marry and under what circumstances. The government has no power to control the religious aspects of marriage. The first amendment prohibits government from making any law that will affect religious practices. If a same gender couple (or any other group arrangement) wishes to have the blessing of a church, synagogue, or mosque, they must convince the religious leaders that their arrangement is consistent with the rules and theology of that particular religious group.
In the final analysis, the state (government) has no compelling interest in making laws governing same gender relationships.