I contend you cannot separate church and state. Even the cornerstone of the Judeo Christian faith advocated to render to Ceasar things that belonged to him and as to divorce his position as that it should be done in a legal manner which were the social and religious laws of society. Most of those laws were formulated by prior lawmakers all the way back to Moses. Many had been perverted over time but the fundamental underlying issue was that even though God established the sacrament of marriage, man determined how to dissolve them and Jesus aligned his own commandment with the rules established with man. Ideally marriage is a union sanctioned by God to last forever but that was dissolved by society early on and now it is so controlled that any rules are almost totally secular.
Yes and no.
While I do agree that the shaping of our society has come largely been rooted in Judaism/Christianity it cannot be completely married together without Theocracy. For the most part America does follow the basic moral laws of Christianity. This country was no doubt shaped by that. While I am a fan of that fact we cannot allow the government to rule the church. When you try to rule the country with the church you are in essence attempting to join the power of both of them and will ultimately surrender part of the power of the church over to the government. Bad idea. Remember, we started this thing to have freedom from government to worship. It seems disingenuous to try to sway the government religiously when we truly should desire to be separate.
It seems that you are using the term "render unto Caesar" incorrectly. That scripture refers to Christians to obey the laws/tax laws of the land. Not that Caesar should rule over religious practice.
It doesn't have to be an assault on our beliefs to let the church be the final authority on our christian marriages. Matter of fact, I'd rather be free to practice my religious beliefs apart from the gubmint. They can take their perks and shove them as far as I'm concerned. If any church wants to marry or deny marriage to any couple then they can find another church or become civilly unioned or whatever they want. You can have you marriage recognized by the government through civil unions and let the church part be a choice. I fail to see how this hurts anyone.
We live in a post christian society. I feel strong about my beliefs in christianity and I think that spreading the gospel is the most important thing a person can do but I refuse to force people to practice my religion through laws. It is not right. Jesus was a gentleman. You can believe but He's not forcing you to follow Him.