In Oct 2012, a Columbia political science professor was charged with incest committed with his 24 year old daughter. The original article is here. As disgusting as that is, what he ended up saying in response to his charge was very interesting:
"It's ok for homosexuals to do whatever they want in their own home. How is this so different?
'We have to figure out why some behaviour is tolerated and some is not.'"
I have come across many people who do not see the implications of their support for homosexual activity and marriage. What this political science professor said was logically correct.
When we as a society wrongly acknowledge that there is any acceptable sexual activity beyond husband and wife and extend that to anything beyond this, we end up with situations like this. What the professor mentions about figuring out why some behavior is acceptable and other behavior is not is very significant. Only a fully consistent Christian worldview would be able to properly articulate and comprehend what behavior is acceptable and which is not.
Also, any reference that simply stops at "natural law" is (as I have said many times) epistemologically deficient and also not fully sufficient from a normative (standard for right and wrong), motivational (why we should right things) or consequential (the effects of our behavior) perspectives of ethical behavior. And as we preach the law and stand for moral truth to those in the public square, at the same time, we also must wisely integrate this with preaching the gospel so that we can offer the lost a way for ethics and ultimately the law of God to not be their condemnation but to be their joy as they worship God.
Challenge any non-married "couple" why extra marital sex is wrong, and any reason that they give will at best from a consequential ethical standpoint, and weak from the motivational standpoint. However, they will never be able to understand why from a normative standpoint extra marital sex is wrong.
In order for ethics to be understood properly, by anyone whether Christian or non-Christian, all three aspects of ethics must be discussed and examined through the lens of the Christian worldview and the gospel.
This can never be done going no higher than natural law as espoused by the (R)2K doctrine.
This cannot be done when reformed Christians absolutely refuse to CONSISTENTLY understand that God's law as reflected in the Word of God is the basis for the normative perspective, it is the loving reason for the motivational perspective and it is the true lens by which we can examine the consequences of ethical actions.
Only through a gospel infused and proper Christian worldview can we understand all of these perspectives on ethics and be able to provide an epistemologically consistent answer to the political science professor as to why his behavior is wrong and what other non-marriage sexual behavior is wrong and how we can know that that other behavior it is wrong.
Reference: "By This Standard", Greg Bahnsen.