Matthew Lee Anderson offers a helpful summary, both of the most prominent “natural law” arguments regarding marriage and of the “evangelical” hesitations regarding them. You can read the Robert George, et al paper here, brief accounts of the Protestant stance toward natural law here and here, and an interesting exchange between Robert George and Albert Mohler here. Mohler gets the nub of the issue right here:
Dr. George and I would argue many of these issues differently. We come from different theological positions, we come from different ways of doing moral argumentation, but I’m always fascinated to have a conversation with him and to see his mind at work. I think one of the crucial points of distinction has to do with just how compelling we believe the natural law to be. At the end of the day Professor Robert P. George really does believe that the natural law can in itself form the basis of a compelling moral argument for such an issue such as sexual restraint. I have to come at this from a position that is more informed by Romans chapter one. When I believe that what we are told there is that humanity is dead set to suppress the truth in unrighteousness and that there is no law written within the heart nor within the role of nature that will keep them from doing what they are determined to do except by the regenerating power of God, the gospel of Jesus Christ. There is a restraining grace and for that I am very thankful and I do not deny the reality of the natural law. I do not deny the fact that that is a part of the restraining grace, but at the end of the day, I am not very hopeful that a society hell bent on moral revolution is going to be held in check by our arguments by the moral law, the natural law. I’m thankful, however, that Robert P. George is making those arguments. I’m thankful that he’s making them better than just about anyone else is making them. And as an evangelical, we have every reason to use natural law arguments, we just don’t believe that in the end they’re going to be enough. That’s where we have to come back with the final issue always being the gospel.
Does Robert George really think that the arguments by themselves are sufficient, that unadorned natural reason is sufficient, not just to provide the ground for a law, but also actually to persuade those who passions might interfere with their reason? Certainly St. Thomas Aquinas wouldn’t rely simply on rational argumentation, but recognizes also at least the compulsory and edifying aspects of human law. In other words, argument always occurs in a cultural and social context, a context that Thomas surely recognizes must be affected by human sin. (An explanation for why I choose Aquinas rather than George here: I may be a poor Thomist, but I’m a better Thomist than Georgian.)
So how much of a difference is there in the end between the natural law of someone like Robert George and the reliance on Gospel and grace of someone like Albert Mohler? I’m sure there are readers who can give better answers than I can.