I've received a couple of questions regarding this matter and basically I can sum up the questions in this way...
Why are so many Evangelical and Reformed leaders so adamant about the need for Christians to oppose the Homosexual agenda, not just theologically, but in social and political terms as well? And why has this become so much more intense over the question of marriage?
It's a complex question, but to put it simply....for different reasons. Since that's not very helpful, we'll have to look at some of the larger issues, or to put it differently, we have to look at some basic or foundational issues and assumptions that are driving people to think a certain way and thus respond a certain way. After examining this a bit, we can weigh their considerations and then attempt to come up with an answer, hopefully one that is Biblical and helpful.
For some of these men it's because the Kingdom of God to them also includes the culture. This is the foundation of Sacralism. The Kingdom includes the elements of culture and thus by necessity the legal and political order.
So to determine if they're correct, this question must be wrestled with...does the Kingdom of God include any given culture? What is the nature of the Kingdom of God?
They will answer that the Kingdom is meant to encompass culture and it is the task of the Church to work toward cultural transformation...to engage the culture and redeem it.
And if the culture doesn't reflect Biblical values then they think the Church is being defeated. They believe it is the task of the Church to work toward making the culture reflect what they believe to be Biblical values.
While on the surface it sounds not only plausible but perhaps even obvious...it is actually a fatal error based on several erroneous assumptions. I probably don't need to elaborate all of them here.
Briefly, from my standpoint some of the opposition stems from a wrong view of the Kingdom. I believe the Kingdom is Redemptive, and Holy. Only the Redeemed are in Covenant with God, and only that which is Holy will survive the Second Coming of Christ when this world will be purged by fire and replaced/reformed with a New Heavens and New Earth. Our cultural endeavours are not sanctified. Christ did not die for or redeem certain political structures, art forms, musical modes, languages, or architecture. These things aren't necessary evil in and of themselves, but they are not actually part of, nor do they contribute to the Kingdom of God.
The Covenant theme running throughout Scripture declares that God will be our God and we will be his people. While Christ is indeed Lord over the world...Creator and Judge....he is not 'their' God and they are not 'His' people.
Hence...Two Kingdoms. One Holy and Redemptive, and One Common, providing a venue of restraint, delay, and mercy until the time of Judgment comes.
Some Sacralists acknowledge that a One Kingdom construct is problematic so they will say...yes, Two Kingdoms, but the law being God's Law is eternal and thus the same for both. One acknowledges the Law....and the other doesn't....and this situation must change. So while they acknowledge Two Kingdoms....the very idea of two is a problem. What the Church needs to work towards is making the two into One Holy Kingdom.
This is to ignore the nature of the Kingdom and the function of the Second or Common Kingdom. It doesn't exist to be transformed. Again, it exists because God is both longsuffering and like with the Canaanites, allowing judgment to fester.
The Second Kingdom is non-redemptive. It is not Holy. It will not survive the 2nd coming of Christ when this world is burned up. It and all it contains will not be present in the New Heavens and New Earth.
The unbeliever does not by virtue of being 'Western' contribute to the building of God's Kingdom. It's not a Kingdom that Michelangelo, Bernini, Bach, Bohr, Friedman, or Wren can build.
If this issue isn't dealt with, this fundamental question concerning the nature of the Kingdom...then no progress can be made. It's pointless to argue with Dominionists over issues like Gay Marriage. We're not dealing with the same assumptions. Until their erroneous view of the Kingdom is corrected, they not only will not accept Two Kingdom arguments, they clearly show that they're unable to even grasp what you're saying.
John 3 is a passage we all know so well, but the profundity of Christ's words is often overlooked....
Except a man be born again he cannot see the Kingdom of God
This is but one of a myriad of verses concerning the Kingdom that modern Dominionists have missed. They read it, but have not worked out the implications. The fact that someone must have the Holy Spirit working on their heart in order to merely 'see' or 'grasp' the Kingdom tells us something very critical concerning the nature of this Kingdom.
It is Kingdom that is not of this world, it is a Kingdom of Righteousness, Peace and Joy in the Holy Ghost. It is a Kingdom that men cannot say 'here it is' or 'there it is'...it is within us. It is one that you must have eyes to see and ears to hear in order to partake of it. It is the power of God at work in lives. It is entering into a state of Justification, Sanctification, and Adoption whereby we call God....Father. He is our God and we are His people.
The Kingdom of Dominionism is the same type of world kingdom, imperial cultural and political structure the Pharisees hoped for. They wanted a Messiah who would make a New Solomon-type Roman Empire. The Dominionists essentially want the same thing...the very thing repudiated by Christ and the writers of the New Testament. The very thing warned against in the Apocalypse.
And as I've said elsewhere, it's not just an issue that we can say...okay, we'll agree to disagree.
The implications of Sacralism....defining the Kingdom in terms of culture and thus political structures.... are not neutral. It's a cancer and I would argue destroys the Church from within. It works this destruction in many ways and on many fronts.
One way....a way pertinent to this whole discussion of homosexuality in our culture is to distract the Church from its true task and mission...by erecting another gospel and engaging the Church in political struggle that in the end binds the Church's hands....it hinders the Body of Christ from being able to go about its true task which is to make disciples of all nations.