Interestingly, religious law seems like a major oxymoron considering that church and state are supposed to be, and are meant to stay, separated at all times. Now, I think your question is asking whether we can combine them. If this is the case, then the answer is that we would have to admit every religious law as part of the deal. If that were to happen, we would be in deep trouble.
Now, what is interesting to me is that the topic of gay marriages has come up as a "religious" issue. With all the advances in psychology and science we know for a fact that homosexuality has existed historically since before the Romans, and will continue to exist until the end of times. It is not something new, it is not something wrong, and it is not something that has anything to do with religion. It is simply a big old elephant in the living room that we continue to ignore.
Gay marriage is a matter of personal choice, not of religious belief. Now, to see the issue from another perspective, if we were to include any religious law and enforce it through civil law, then we should start by charging every indigent and irresponsible pregnant woman out there with a felony for the future endangerment of a child. I sure would have taken a couple of parents into custody by now. And that would be the morally right and righteous, virtuous and truly "Christian" thing to do, as we say in the South.
You might also like to consider the practical issue of medical intervention to save lives. Some religious groups do not allow such medical treatment as blood transfusions, which have resulted in a conflict between civil and religious law in the case of children needing such treatment in order to survive. What do we do in such a situation? In Britain, at least, it has been legislated that in the case of such minors, parental views should be overruled and children should be given the medical care deemed necessary to ensure their survival. This is an interesting case however that gives us a real life or death scenario and explores the conflict between religious and civil law.
I think you are missing an important distinction. If I choose to live my life according to a set of religious beliefs, I am choosing to accept the directions and requirements of that belief as law by which I will structure my activities. This does not mean that you also have to follow the guidelines which I am adopting, but it makes them religious law for me and for others who adopt the same faith.
The Constitution created a separation between church and state because the Founding Fathers recognized the difficulties that developed when either of those institutions tried to influence the workings of the other. Unfortunately, saying that they are separate does not guarantee that their spheres of influence will always remain separate.
Other previous posts have highlighted areas in which conflicts occur and tough decisions, based on the circumstances of individual cases, need to be made. In my opinion, there are going to be more of these kinds of situations in the future as questions of cloning and medical technology create more bioethical issues for consideration.
Divorce has been mentioned several time in the previous posts. Marriage is a dual system, it is not only a religious covenant but also a legally binding contract between the spouses and the state. It is for that reason the state involves itself in divorces; it was a party to the original contract.
More to the point, our Constitution prohibits interference with the Free Exercise of Religion, but also prohibits an Establishment of religion. The Supreme Court has interpreted this to mean there is a "wall of separation" between Church and State--a phrase first used by Thomas Jefferson. Religious law and civil law may easily coexist, unless there is a conflict between the two. In that instance, one would necessarily need to examine the rights of the person practicing the religion to the rights of others as well as society as a whole. It is doubtful that civil law would interfere with a sacrificial rite, unless that rite involved the sacrifice of a human being, in which instance the protections of the 5th and 14th Amendments would preclude its practice.
This is a difficult and contemporary question as there are currently places in the world where religious law and civil law are separate entities and are often contradicting or in conflict with each other. Historically, as was the case in the Roman Empire regarding Judaic religious law, civil law often had the final authority, yet may have exercised it with care since the populace would have held strong feelings and had strong convictions about overriding or disrespecting--what they perceived as supreme--religious law.
Therefore historic records often cite extreme conflict when religious law and civil law oppose each other. An example might be when the religious law of the early Christian Catholic Church decreed crusades against Muslims in Jerusalem and Crusaders violated civil law by massacring Jews in Jewish communities throughout Europe as the Crusades made their way to Jerusalem.
There is room in a civil legal system for tolerance of religious laws, even if they are outside of the societal or governmental norms for behavior, but not room for full authority and autonomy of religious laws and governments.
Social and moral law can for the most part coexist with governmental laws, because, in most ways, they govern different things, and religion is a choice for Americans. Many civil laws reflect beliefs that fall along religious lines, or were passed in an earlier, more religious time, and therefore reflect the same moral values.
This is, of course, a matter of opinion.
My opinion is that the civil legal system should stay out of decisions about religious law except in cases where a party claims that the religious law violates civil law. In such cases, civil law must be supreme and must override any religious law.
For example, if two people choose to get married by a church (and not register their marriage in a civil fashion) the government should not get involved if they wish to divorce. Their marriage should not exist in the eyes of the government. The government should only step in if, for example, one partner is claiming abuse and the religious law holds that the abuse is acceptable. In such cases, secular law is being (potentially) violated and the government must step in.
I think the first thing to say is that there is no such thing as religious law. Not in The West anyway. Just because a religious text makes legal-sounding pronouncements, they are not laws. I can think of two recent examples to prove this.
In Holland it was recently codified into state law that the ancient practices of Halal and Kosher animal slaughter are illegal and must cease. End of story. No special exemptions. Killing animals in a cruel and unusual manner is not 'OK' just because a desert nomad 3000 years ago said that a god told him to do it.
In another example, in the UK, The Catholic Church's adoption service refused to consider gay couples as adopters and the church justified this exclusion based on prejudice found in the Bible. A judge slapped them down and quite rightly said that institutional prejudice is simply not acceptable, end of story, and found them guilty.
There is no such thing as religious law.