Well I initially thought I would not post on this since this is a dying issue for the Federal Government. They just took the issue off the floor of Congress failing even to meet the number of votes they had two years ago when this topic was approached. It will end up in the courts for each state until it reaches the Supreme Court to make a ruling where they will likely uphold the status quo and leave it up to the states to make the decision themselves.
Personally I think it is a religious issue divided among party lines and the Republicans will be using it to keep the conservative voters in their corner as dissatisfaction grows with the President. The message they will send out during the next election will be that they either have to support Republican leadership no matter how bad they are screwing up or support gay marriage.
It is a religious issue in the sense that conservatives call it protecting the "sanctity of marriage." I suppose they protect marriage's sanctity by allowing you to be married by the variety of freaks out in Vegas such as Elvis impersonators, aliens, drive through windows and the like. Yep, that's traditional religious values I suppose and needs to be protected.
Marriage is not a federally regulated social phenomenon and for them to step into the arena is a bigger issue of stepping on states rights to govern themselves. Sure, the federal government regulates Native American marriage under Title 25 CFR, federal prisons, immigration and the like but not who constitutes man and wife with one exception and that is Veteran Affairs. Title 38 CFR"Spouse. ``Spouse'' means a person of the opposite sex whose marriage to the veteran meets the requirements of Sec. blah blah blah..." This kind of throws out the "don't ask don't tell" policy of the military if a same sex marriage took place between a same sex couple after military service since the surviving same sex spouse would be denied a death benefit.
There are numerous legal concerns over same-sex marriage such as states have to accept the marriages performed in other states, protection by the law that married couples get, allowing sodomy (currently illegal) and tax status among many others. So allowing same sex marriage in one state and not another would be a hassle but not impossible. The majority of the concerns proposed are "moral" or religious in nature and have nothing to do with the law itself. The debate, when it hits your state's legislature, needs to be defined honestly. Is it a legal issue or a religious issue for you and if it is a legal issue then keep "sanctity of marriage" comments out of the debate. If it is a religious issue then it should not be in the law so as to keep a separation between religion's idea of what is right and wrong out of the law. The choice is up to you come voting time.
The Establishment and Free Exercise Clause is what will be the main issue for the courts to decide upon. They really need a good definition for this and historically it has been all over the board, both narrow and broad. The broad meaning is to forbid the government to financially support a national religion such as the Catholic Church but they have financially supported Christian missions to the Native Americans. The narrow meaning, such as those described by Jefferson and Madison, is to protect the freedom of the individual and keep a wall between church and state. See http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/estabinto.htm
If the Sanctity of Marriage is the main concern and not the state's right to govern themselves or legal issues surrounding same sex marriages; then the Supreme Court needs to make a firm decision between narrow and broad interpretations of the Establishment and Free Exercise Clause for this issue and many others to make headway in any direction. One of the religious arguments against same sex marriage has been that it will lead to polygamy, child molestation, restrictions on the religious, threaten their tax-exempt status (I'm for that), and other things even more freaky. But of course the same could be said for not allowing gay marriage in that it could lead to more control over our lives by the major religious institutions on what is permissable and not. It could lead to every theater being allowed to only show The Sound of Music and book burnings. Personally, I think that gays should form their own religion and get thier benefits that way. It would be less of a hassle in the long run than trying to overthrow the power of the Religious Right. If you can't beat 'em, join 'em.