My thinking on natural law goes back to John Joe Lakers's position that human morality is governed by one of two metaphors: the metaphor of power and judgment, and the metaphor of intimacy, which he defines as passionate, respectful, vulnerable, faithful involvement of one person with another, or of one person with God, or of God with us. Intimacy is another word for love.
The metaphor of power and judgment is clearly the more dominant of the two. It can be found in much of "Old Testament" scripture, and certainly in most of our criminal law. You judge whether something is right or wrong, and if it is wrong, you punish it. This suggests a behaviorist assumption of how humans operate. We act on the basis of punishments and rewards.
JJ, in the months and years before he died, carried on a sustained attack on natural law theory. But in a sense, he is really stating that each of his two metaphors is "natural," in the sense that humans naturally operate on their basis. Therefore the attack should not be on natural law as such. There is indeed a natural law, but it has both positive and negative aspects.
Negatively, one could make the case that human beings are structured naturally to dominate each other, even to the point of slavery, which is what slave-owners argued. Or that men are naturally sexually promiscuous, which is what some argue today. Or that men are naturally dominant over women, also argued today. We have learned, I think, that when a woman leads a professional life, she is not violating her nature.
On the positive side, we can argue that intimacy is "natural." When one lives by that metaphor, good things happen. I might argue that humanity might never have learned the value of that metaphor had not Jesus taught it, but it might also be true that people have indeed discovered it outside of Christianity. There is no doubt, however, that for Christians, it should be dominant. "Love one another as I have loved you" is how Jesus put it. Or, "The greatest commandment is this: you shall love the Lord your God with all your heart and with all your soul and with all your strength. And the second is like it, you shall love your neighbor as yourself."
That intimacy can be the basis of a Christian ethic is clear enough. We get into trouble, however, when we start to define specific behavior as "natural." Is it any more obvious that contraception is unnatural than that slavery is unnatural? Or that in vitro fertilization is unnatural than that male domination is unnatural? Questions like these are the reason that most social scientists reject the idea of anything being strictly "natural." For them the nature-nurture debate colors every discussion of what kind of behavior is good for human beings. To say that something is natural is to end discussion, and ending discussion is something that a scientist will not do.