Marriage is an institution of God. For those of you who don't believe in God, it is an institution of religion. Therefore, I say it is un-Constitutional for government to endorse or engage in the administration of traditional marriage.
It has always been in a government’s best interest to encourage and reward people who choose a life of sexual monogamy and financial singularity (aka "marriage"). I won't go through all of the benefits to society, but they are many and certainly worth reward. The question is, in its "best case scenario," does homosexual marriage benefit the country or even the populous? My answer would be (discounting the negative effects) yes; there are some societal benefits to sexually monogamous and financially singular homosexual relations (arguably less than traditional marriage, but benefits nonetheless).
Establishing that sexual monogamy and financially singular behavior between two people are of benefit to society, it is the government’s job to incentivize such a voluntary commitment. Does the government have the right to use the word marriage? Marriage traditionally, has been defined as a life long oath, sworn to God, between a man and a woman. The sworn oath is different from religion to religion. Is it government’s job or right to provide administration for God? I don't think the Creator of the Universe needs much help in that regard. The interconnection of marriage and government comes from a day when the government was for all intents and purposes, property of a religion; the U.S. is not.
So today we argue on homo/hetero marriage. There should be no argument. Actually, both atheist and believer should, for different reasons, be fighting the governmental use of the word marriage. To an atheist, the use of the word marriage carries religious connotations. To a believer, use of the word could be considered heretical. Particularly when the government itself can not by its Constitution, make a law establishing religion (endorsing a particular God/god).
Here is my proposal in a short context. Get rid of government defined marriage. Marriage belongs to God not government. However, the government should endorse and reward a contract between two. The contract should be straight forward and direct. I would call it the, “Contract of Sexual Monogamy and Financial Singularity” (CSMFS). Breaching the terms of the CSMFS, would result in forfeiture of all future government rewards or entitlements. Like any tax deduction, shelter or tax credit, breaching the terms of the CSMFS (such as promiscuity), would result in certain punishments that would start at simple loss of privilege and possibly grow to tax evasion and imprisonment. Like any contract, if you agree to terms in order to reap a benefit; breech should also carry consequences.
Marriage in America is a joke. It is a pledge sworn in front of a judge with no consequences. The government SHOULD NOT be in the business of administration of God's tenets.