There are three preliminary conclusions which influence everything else I believe on this subject, (the role of religious law and belief in civil government) and without agreement on which I couldn't really debate it. They are:
1. Involuntary acts mean nothing in terms of morality.
While actions may be good for us and good for society and generally a good idea, no action pleases God unless it is done with the right motive and is voluntary. If you treat people kindly because someone's going to hurt you if you don't, it's still treating people kindly, which is an inherently positive thing, but it is very different than treating people kindly because you love God and you view that as part of your faith in action. In both cases you have done something good, but doing it because you choose to is different from doing it because you're forced to. God wants love and unbounded choice to do good, not just lip service and exterior forms of godliness.
2. We cannot, and we should not, try to coerce conversion or religious belief.
To do so would be morally wrong. We are to witness with service, example, and sharing/teaching, but we are not to bully or shame. Ever.
3. It is not the government's job to legislate morality or to shepherd the faith of its people
It is the government's job to protect the freedoms and general of its people, to enforce the rule of law, and to facilitate infrastructure in as local as way as is feasible.
That said, I absolutely think Christians have a responsibility to be involved in our government at all levels, in the arts, and/or in whatever ways we have been gifted and enabled to charge our world for the better. Part of our mandate as Christians is to release the captives, comfort the mourning, and bring justice to the oppressed. We can do this through politics, and as it is my belief that a free society is most conducive to this, working for a just and free community can be a big part of fulfilling that mandate. We may also have opportunities to share our faith with those who ask and who we would not have met otherwise, and to witness by our example of service to our communities. Working in politics to make our home free, prosperous, and friendly to the free exercise of our religion is a great and noble work. However, this does not mean that our laws should conform to any specific set of religious dictums. As I mentioned in a previous post, there are many problems inherent in an ecclesiocracy/theocracy, such as lack of agreement within faiths, a substitution of rule books for divine relationship, etc. As a Christian, I believe I have a responsibility to foster a political environment that is friendly to my faith and does not prohibit it, but also does not mandate it or interfere with it unless it harms others or endangers their basic freedoms. I view separation of the state from any one church as an inherently good thing. Of course, the state will be influenced by the faiths and churches of the people who comprise it; after all, what is government but the people to we elect to do the jobs we cannot or do not wish to do? But- to call ourselves a "Christian Nation" can be a bit problematic. What does that mean, exactly? With which flavor of Christianity would we be identifying? Southern Baptist, Anglican, Methodist, Mennonite? If by Christian Nation we mean a place where Christians and other can freely practice their religions with a few caveats, or a place where the basic values taught by Christ in the beatitudes are a fundamental part of the legal foundation, then I think we can proudly own the term. But if that means that being an American is synonymous with being a member of the Christian faith then I think I'd rather call it (and I can't at this time, not completely) a nation of Religious Freedom, and a nation whose people take responsibility for it.
All that said- what should be our foundation for law? Our constitution is based on a value for liberty and human life, and on justice and general ontological equality. I cannot think of a better starting point. Those things are also part of the Christian faith, but are not necessarily unique to it; many atheists would hold them as a positive foundation for a legal system as well. To hold up such a standard does not interfere with religious freedom, as our constitution is not part of any religion. It contains theistic statements, but I think it is generally understood that theism is not required for citizenship or to uphold the values set out in our founding documents. There are some things required of certain religions which violate our basic legal framework, and that is where religious freedom should end- take honor killings, forced marriages, or state punishment of consensual adult sexual behaviors such as adultery, premarital sex, or homosexuality, for example. We could always privilege the Christian faith, but that would be almost impossible due to the diversity within it. I firmly believe that the basic values of our constitution are thus a better framework for our government, and will more likely create a state of religious freedom, than any strictly religious law (Levitical, or Sharia, et c) or religiously mandated form of government.
The fact that our faith requires a position or behavior from us is not sufficient to legislate it for those who do not share that faith. Most of the dictates of the Christian faith are within the "value for liberty and human life, and on justice and general ontological equality" category, and thus are law not because they are in the bible, but because they reflect our basic legal ideas. Everything from insurance fraud to murder to the abortion debate can be traced back to those basic things. There are, of course, some things that many Christians believe to be morally wrong that do not fall within the confines of proper governmental jurisdiction. Adultery and Homosexuality come to mind- we do not throw people in jail for either one, nor should we. The state is not our parent, and we are not its child; consensual behavior between adults does not negatively affect the general population in a way that merits government interference, and all moral and religious implications are the purview of the parties involved, their church, and their God. There may be consequences- in the name of justice, a spouse may seek a divorce when adultery comes to light, and because marriage involves a contract which in such a case was broken, the offended party may be entitled to a distribution of assets that reflects this. This is very different from jailing the offending parties for their moral sin, however.
There is no reason for the state to discriminate based upon a religious rule that is not necessary under the ideals of the constitution. For example- many Christians believe homosexuality is morally wrong. Still, the state recognizes heterosexual spouses and parents regardless of qualification, and does not (and should not) require that homosexual couples should have to prove their fitness to acquire children any more than should heterosexual couples.
(Also, as a matter of consistency- it makes no sense, biblically, to limit homosexual marriage any more than we limit adulterous marriage. Let's be consistent- if LGBT folk can't really be married, then neither can people in polyamorous relationships or people who cheat on each other. Not the government's business, you say? Mmmm..... exactly.)
If we wish to make a law prohibiting marriage between gay couples, it is then inconsistent to use religious or biblical grounds to do so. If we desire such a law, we should first determine whether same sex marriage violates the principles of liberty, respect for life, justice, and equality. If same sex marriage is not counterintuitive to those things, then we ought to ask whether or not it does quantifiable harm to people outside the relationship and infringes on the rights of others. If it does not, then, irrespective and regardless of its morality, I question the legal justification for such a law.
Because involuntary morality does not count towards true holiness, we are doing our society and the people in it no favors when we legislate religious observance. Making laws which render our home freer and safer is wonderful, but legislating religious morality or observance of biblical law which falls outside of our constitutional purview simply puts our people in a place of observing law from fear of punishment, rather than because they are attempting to love God more fully. (I am in no way saying that I believe Christians are bound by the Levitical law, by the way. I'm just using it here as an example of an extra-constitutional morality code that we could, but should not, adopt.) I can see no end to such a course but useless bondage.
Some may argue that allowing same sex marriage necessitates a fundamental redefinition of marriage. To that, I'd say- 1, that depends upon your definition of marriage, and 2, the definition of marriage is not specified and hallowed in our constitution as are other issues. Marriage, while it has historically been heterosexual, has not historically been a Christ-honoring, mutual, monogamous relationship, even within the church. There are exceptions, of course, but unless we're going back to the garden of Eden before misogyny reared its ugly head, then we're dealing with a mixed bag that includes a healthy dose of polygamy and a heaping cup of gender inequality. I really see no problem with clarifying its legal definition to include committed monogamous relationships between any two humans.
Then, of course, there is the issue of civil vs. religious marriage- I would support a state-recognized civil union between any two people, and the idea of leaving the religious end of marriage the purview of individuals and churches. That's a rabbit trail for another time, though.