I just read this essay over at Mere Orthodoxy and it agrees strongly with my personal experience. The summary is that many Protestants have risen to the challenge of defending natural law theologically, none are optimistic about its persuasiveness in the public square. This should be obvious since the public cannot tell the difference between male and female, much less what oughts we should derive from what is. If Westerners from a Christian civilization do not find these arguments persuasive, how much less will non-Westerners find them persuasive? How insufficient, therefore, is natural law as a basis for relations among nations?
To summarize, while Protestant scholars are increasingly open to the idea of natural law, many register a vote of “no confidence” when considering the question of how persuasive we can expect natural law arguments to actually be. They are hardly alone in making this assessment either, for many Roman Catholic scholars of natural law have expressed similar sentiments.
Let’s look at this practically. Westerners believe in Hume’s “ought/is” distinction: you can’t derive ought from is. Just because you have a brain doesn’t mean you ought to use it. Just because you’re male and larger, stronger and more athletic than your wife doesn’t mean you ought to defend her from an intruder, and so on. Further, you can’t tell a Westerner to use the brain God gave him to study or to use the limbs God gave him to work since he doesn’t practically believe God gave these things: we’re just evolved from a common ancestor of apes. Natural law, in the Western tradition, hinges on Creation, man made in the imago Dei, and a telos for nature, including man. All of these things depend on special revelation (the Bible), not general revelation (Creation). This should pound the nails into the coffin for natural law as a basis of public morality or reason in the West. So what? Be pessimistic about the West as a whole but try to build Christian community on a local level. Remember: reality wins in the end whether or not the Left denies it exists.