There is a terrible irony in the fact that many ancestors of Americans fled to the new world to escape religious persecution yet their descendants are still fighting the same battle. When I was growing up, I was a Christian. Today, it seems that all you have to do is say "I'm a Christian" and whether or not you follow the tenets of Christianity is almost irrelevant. There are many good, decent Christians out there. However, in the crisis the US faces today, they've been largely silent. I firmly believe that silence is agreement, so when the Christian Taliban tries to take over our political process and the masses of Christians remain silent, I blame most US Christians for this evil.
The following is presented with the view of Christianity I held when I was a Christian. Much of the following is just from memory, so my apologies if I get little bits of it wrong.
When the Pharisees presented Jesus with a coin with Caesar's image and asked if he should pay taxes (tribute) to Caesar, they presented him with what appeared to be an insoluble dilemma. If he said "no", then the Romans could swoop down on him for advocating violating the law. If he said "yes", his followers could accuse him of putting Caesar above God since the coin proclaimed Caesar's divinity.
However, Jesus neatly side-stepped this with his famous "Render unto Caesar that which is Caesar's and unto God that which is God's" (that might be off slightly, but it's basically the same thing). What Jesus said made it clear that separation of Church and state is something which Christians do not have to reject. A state must necessarily accept that the people in its borders often have a variety of differing views. The role of the state, in this case, is to ensure that the expression of those views does not demonstrably harm the people over whom the state has jurisdiction. While this clearly has grey areas, the state is perfectly within its rights to prohibit murder, theft, rape, and so on. At no point in reading the Bible do I remember Jesus saying that a state had to enforce religious beliefs.
Some Christians in the US want their personal views encoded into law. However, they ignore some pretty salient points of the Bible. Remember when Christ protected the adulteress from being stoned? Why did he do that? I think many fail to consider this.
However, it goes back even further. If God did not want people to have the ability to choose, he wouldn't have put the Tree of Knowledge of Good and Evil in the Garden of Eden. By putting it there, he was making it clear that Adam and Eve had the right to choose whether or not to sin. If one is to be a Christian, what hubris compels them to deny a right which God granted?
More immediately, let's say that you just lock up everyone who sins. Does physically preventing someone from physically doing something you consider sinful make them a better person? No! It merely prevents them from doing something you consider sinful. As a result, laws designed to prevent sinful behavior, while they might protect society, don't do a damned thing in the eyes of God. Instead, people should be allowed to choose their path so long as the only people affected are consenting adults. If they go to hell, that's their choice, as given by the God of the Bible. I can tell you not to steal from me, but I have no right to tell you it's illegal to participate in consensual group sex.
This is the problem I have with today's Christians. The god I read about in the Bible let people choose whether or not they wanted to be sin. Of course, there are still difficult bits like "never suffer a witch to live" (there are differences of opinion over that interpretation), but particularly with the New Testament, it was strongly reinforced that we don't have a right to judge morality. That's God's right. We have a right to protect ourselves from someone trying to murder us, rape us, steal from us, etc., but their personal decisions of right or wrong are not our place to judge. That's why I firmly believe that if there is a Hell, Falwell and friends will be roasting in it.
If you're a Christian and you think you can legislate your beliefs, how dare you try and force me to do something even your own god won't?