Not quite. The gov't can't create a state religion. It doesn't say that the laws can't have a religious basis. Blue Laws, which exist to the day have a religious basis and are still Constitutional.
I was following you until that last part.
The State makes laws regarding marriage because the electorate either wanted them to or allowed them to. This is, after all, a government of "the people", by "the people" and for "the people". If the majority of "the people" want certain types of laws, they get them. Right now, the majority of "the people" don't want marriage redefined.
There is no monolithic Christian Religion as far as I know. There are a collection of Christian ReligionS, such as Baptist, Catholic, Methodist, Quakers, etc., etc., and the government doesn't make any law determining which particular flavor is acceptable or not, therefore, no State Religion.
Religion is one of the bases of morality and morality is one of the bases of law. It is impossible to divorce the two, IMO, without losing the common link, morality.
It is up to society to decide what is and what isn't socially acceptable standarRAB of behavior. Just as local communities can decide if nudity is acceptable for its citizens, on a more macro scale, it is acceptable for society as a whole to determine what the qualifications for granting a marriage license are. After all, other licenses granted by the state have restrictions that have been found to be constitutional, such as driving age restrictions, liquor licenses, etc.,