The government has no legitimate business promoting (by tax relief or health benefits or inheritance law) nor licensing any domestic arrangement of persons who enjoy each other's company at that moment in time. It is, as many are eager to point out, and have already pointed out, a private matter between the individuals.
The government has every legitimate right and duty to promote (by tax relief, health benefits and/or inheritance law) and to license stable nurturing environments for the raising up of the next generation of citizens, with the goal to make them productive members of society. Whether we give this environment the label "marriage" or some other name is quite beside the point.
Generally speaking, and assuming that the system is functioning correctly, the two persons with the greatest a priori personal stake in properly raising the children who are the next generation of citizens, are the biological mother and father of each child. Therefore it makes sense for the government to work with (as opposed to against) the forces of nature by providing governmental incentives for those two biological parents to remain together and in support of those children during the period of their immaturity. This the government can do by promoting the idea of an enduring marriage between a man and a woman, and by discouraging childbirth outside that marriage. Societies and governments have successfully done so for thousands of years.
While making appropriate allowances for unusual circumstances, the norms and laws of society should not be determined primarily by the failures. Thus while some people are inclined to murder and steal, the laws are designed to protect the lives and property rights of the people who do neither. Similarly, while some married people may develop abusive relationships, the laws should protect those who do not. Moreover, since abuse as a behavior pattern is the choice of the abuser, laws can be designed to discourage it as they now discourage murder and theft , while still maintaining a stable environment for the children.
Without upsetting the main intent of encouraging biological parents to raise their own children, the laws may find it necessary to protect children from incorrigibly abusive parents, or to provide alternative accommodations for the children of parents in prison or deceased. Usually the biological next of kin (parents, brothers and sisters of the natural parents) are similarly most strongly motivated to raise their own near family members, and should be encouraged to do so. Failing these first-line measures, foster parantage or adoption are viable alternatives -- but these options should not be considered first in the best interests of raising children.
Where does homosexual marriage fit into this picture? It doesn't. Apart from unethical scientific experimentation with unnatural cloning techniques, homosexual partners cannot become the two biological parents of a single child. Therefore their union is almost never the best nurturing environment for any child, and the government has no business setting up marriage and faux-marriage opportunities to encourge them in their unnatural  desires. Furthermore, the state has a legitimate secular purpose in discouraging homosexual behavior (at least in males, as practiced), because of its unhealthful effects and consequent burden on the public health system .
We note that current marriage law in the USA is not optimal with respect to its valid social purposes; no-fault divorce is probably the biggest detriment, but the diminished social stigma of illegitimacy and the widespread use of contraceptives to implement DINK (double income, no kids) also work against a proper understanding of what marriage is and should be.
First Draft: 2004 October 4
2. For those persuaded by Darwinian theories of natural selection, it is easily seen that homosexual union produces no offspring to carry on the genetic line of those practicing it, so therefore natural selection is effective and immediate in determining such activity "unfit" in the sense of "survival of the fittest;" there are no survivors in the next generation from homosexual union. For those unpersuaded by Darwin, they will find comparable teaching in their own religious traditions, which are not the subject of this essay. All of science and history agree on the unnatural quality of homosexual activity.
3. It's unfortunate that the state of Texas
chose not to defend their sodomy law on the basis of the burden it places
on the public health system. With such a defense, the Supreme Court probably
would have upheld it, just as courts have consistently upheld motorcycle
helmet laws when defended as a public health issue, while striking them
down in states where the major argument is the personal safety of the cyclist.