For example, the Catholic Church opposes homosexual acts, masturbation, and contraception using Natural Law Theory. The reasoning goes something like this:
1.) Nature shows us that sex is for procreation and uniting spouses.
2.) Any sexual act that separates these unative and procreative properties is wrong.
3.) Homosexual sex, masturbation, and contraception are all morally wrong because sex is being used for pleasure alone, denying the procreative aspect of sex.
I have deconstructed the minor premise elsewhere, by explaining how Catholic ethics inconsistently uses categories associated with body functions in other pleasurable activities, such as wine tasting. In this post, I'll focus on the major premise (1), in particular the problem with the word "nature."
Although Natural Law theorists, use the words "nature" and "natural," they use these words in a metaphysical way, consistant with Platonic and Aristotilian science, but ignoring scientific revelations of nature from Newton onward. This makes conversations with Natural Law proponents difficult for those of us who want to bring in information about modern biology and physics into these discussions.
For example, rather than directly address the problems that the theory of evolution, particularily polygenism, may present to the idea of original sin, Natural Law proponents, such as Edward Feser, would rather you think about the metaphysics:
After all, the question of human origins is not a matter to which biological considerations alone are relevant. Metaphysical considerations are at least as important -- indeed, they are more important...
And when it comes to moral issues such as sexual mores, biological data or anthropological data is thrown out in exchange for what must be, or its fittingness. This was the same logic used by geocentricists. Man is the highest of the created order, and the earth is man's domain. Therefore, it is fitting for the earth to be the center of the universe (an for Jerusalem to be the center of the earth).
Unfortunately for the Church, nature is indifferent to metaphysical fittingness when it comes to reality. In fact, modern physics shows us that reality often defies our expectations and behaves in ways that are unexpected. The Heiseneurg Uncertainty Principle immediately comes to mind.
In the same way the Catholic Church clung to a geocentric universe, despite Galileo's evidence to the contrary, it also clings tightly to an Aristotelian understanding of reality regarding biology and moral principles. The Catholic Church will not look at what biology tells us regarding nature because this goes against the predetermined teachings on sexual morality. Even when Natural Law ethicists acknowledge that humans evolved in non-monogamous relationships and have always had sex primarily for please, rather than reproduction, they have an "out." Any evidence from biology that disagrees with their primary premise will be attributed to human "sin."
But where did sin come from if biology shows more than one original Adam and Eve? Well, ignore biology in exchange for a metaphysical explanation...circular reasoning, anyone?