Christians who argue for libertarianism often miss two important points: First, that the Bible does give the civil magistrate authority to legislate morality and that all laws legislate morality.
The most obvious Biblical example Romans 13, which sets the example of the good civil magistrate as one that rewards the good and punishes evil. God gave the sword to the civil magistrate for this purpose. The distinction between good and evil is a moral one and punishing evil is legislating morality. The civil magistrate is given authority directly from God, so we must obey the law unless the government orders us to sin.
Furthermore, it is fashionable to say that “we cannot legislate morality” in response to moral issues, but we need to be clear: All laws legislate morality. Environmental laws enforce a moral code that we do not excessively despoil nature and speeding laws (and other traffic laws) enforce a moral code against us recklessly endangering another person’s life or property. The entire apparatus of the welfare state is based on a moral decision to confiscate the wealth of some to provide for those who are in need.
The question, then, is not whether to legislate morality, but what morality to legislate.
Since we will inevitably legislate morality as long as we have a government, the proper distinction is what is a sin and what is a crime. Being greedy is a sin, but is not necessarily a crime. If greed causes someone to steal, then it becomes a crime. The same is true with hatred: It is a sin to hate, but it is not a crime that is punishable by the civil magistrate until that hate causes someone to commit assault or murder.
So while the government deals with crimes, it is the church and the family that deals with sins. Because the potential for abuse of power is high (and because even a largely righteous government is made up of people wholly corrupted by sin) we should be very clear in what is and is not a crime and the definition of “crime” should be limited and easy to understand.
Ultimately, saying that “we cannot legislate morality” is an extremely childish argument. It sounds hip and enlightened and it can be a cool catch phrase, but it is a simplistic worldview that falls apart when closely examined. We need to be better than that, both as Christians and as citizens.