There's an interesting observation about natural law made by Christopher A. Franks of High Point University in "The Usury Prohibition and Natural Law: A Reappraisal," published in The Thomist, October 2008:
For Aquinas, appealing to natural law meant recognizing our ontological poverty and confessing that the order thus written into our nature and the nature of the cosmos is a manifestation of God's providence.
In modernity, however, natural law became a way to ground moral claims in a language that apparently appealed to humanity as such regardless of one's religious commitment or moral tradition.
He's using the term "ontological poverty" to "refer to the lowly neediness of creatures whose existence is not their possession, but a gift."
And that sounds about right. I often see it suggested that this or that precept is part of natural law, and therefore accessible to all people of good will, without reference to specifically Catholic, or even theistic, principles.
But if there's a natural law written in our hearts, there must be a Writer, Who must have had some reason for writing what He did and not something else.
If speaking just in terms of "natural law" nevertheless implies a supernatural Lawgiver, then maybe natural law isn't such a great way to appeal to humanity as such regardless of one's religious commitment or moral tradition.