In the Catholic natural law tradition initially dominated by St. Thomas Aquinas, there has been a recent formulation over the last half century called the “new natural law” that functions apart from a metaphysical basis; hence, it is a secular theory.
One of the most famous defenses started with Germain Grisez’s “First Principles of Practical Reason” (1965) in The Natural Law Forum (vol. 10) that included a commentary on Aquinas’ Treatise on Law (Summatheologiae 1-2, question 94, article 2). The traditional understanding of natural law – as Aquinas understood the term – was the participation of the eternal law in the rational creature. Divine law was the reflection of the eternal law through God’s revealed Word, and human law was the particular application of natural law to circumstances in human societies.
New Natural Law, in short, draws upon a number of core issues that pertain to what is known as “practical reason” (among other things, of course). In the “old” natural law there are certain precepts (three to be exact) that include primary, immediate, and common precepts. Primary precepts are also known as “first principles of practical reason,” which is something addressed at length in Grisez’s paper (168).These precepts are those “… moral principles that we can’t not know” (Budziszewski, 61). For instance, good should be pursued and evil avoided.
In the “new” natural law, these theorists hold that there “basic human goods” which are self evidently true. For instance, according to Finnis 2011, things such as life, knowledge, health, religion, marriage, and so forth are all considered basic human goods. According to Christopher Tollefsen’s paper, “As grasped by practical reason, the basic goods give foundational reasons for action to human agents. Moreover, they are recognized as good for all human agents; it is equally intelligible to act for the sake of the life of another as for one’s own life” (see PDF here).
In Application to Marriage
One recent and notable defender of new natural law is Robert P. George, who in his book What is Marriage? (2012) argues that marriage is a basic human good that can only be obtained through the procreative/reproductive unit between a man and a woman; hence, homosexuality runs contrary to this basic good.
To start, marriage is a comprehensive union. This means that marriage, unlike a simple companionship or friendship, differs not merely in degree or like “two amounts on two different checks” (to use George’s et al. phrase), but rather contains a deeper human reality that brings together a man and a woman into a “sexual union” (to borrow from Gallagher 2012) – union through body and mind – thus forming the reproductive unit that makes them mother and father, having a prioritized care for the needs of children. This is what George et al. calls the conjugal view.
Budziszewski, J. Written on the Heart: The Case for Natural Law. 1997. Downers Grove, IL: InterVarsity Press.
Corvino, John, Gallagher, Maggie. Debating Same-Sex Marriage. 2012. Oxford: Oxford University Press.
Finnis, John. Natural Law and Natural Rights. 2011. Oxford: Oxford University Press.
George, Robert P., Girgis, Sherif, Anderson, Ryan. What is Marriage? Man and Woman: A Defense. 2012. New York, NY: Encounter Books.
Grisez, Germain. “First Principles of Practical Reason.” 10 Natural Law Form (1965). Notre Dame, IL: Notre Dame Law School