Why Human Nature is Important When Evaluating Sexual Behavior

The Catechism of the Catholic Church teaches that homosexual acts are acts of “grave depravity” and are “intrinsically disordered” because they contradict the natural law (CCC 2357).

The natural law, which is a moral law built into the nature of man discoverable by the natural light of human reason, has always been the basis for the Church’s teachings on sexual morality. But many deem this reasoning unworthy.

At a Human Rights Campaign dinner on October 10, 2009, President Barack Obama called such views “outworn arguments and old attitudes” and said our attempt to outlaw so-called same-sex marriage is an attempt to “enshrine discrimination into our Constitution” (www.whitehouse.gov, October 11, 2009).

Obama’s view is a common one among those who are critical of the Church’s position. Therefore, it’s necessary that we give a rational defense of the appeal to the order of human nature (the natural law) for determining appropriate and inappropriate human sexual behavior.

Instead of asking Tina Turner’s question, “What’s love got to do with it?” we must ask, “What’s nature got to do with it?”

Human nature and our good

The first and most fundamental reason we must appeal to human nature for determining appropriate human sexual behavior is that living in harmony with human nature is constitutive of human happiness.

As I explained in my blog post “The Natural Law: A Guide for How to be Human,” what is good for man is the achievement of the specific ends toward which man’s nature directs him. (“Nature” here refers to the essence of what man is as a rational animal, an essence that all human beings share. It does not refer to what an individual happens to feel or what commonly occurs in the ordinary course of things.)

Consequently, human flourishing (or happiness) is contingent on whether man orders his conduct toward the attainment of those ends. And since our sexuality is a part of human nature, it necessarily follows that our happiness is contingent on whether we live in harmony with what nature demands of our sexuality.

To President Obama, I ask, “What’s so worn out and old-fashioned about encouraging people to live in harmony with their nature as a human being? What’s so discriminating about encouraging people to live in a way that will help them flourish as human beings and achieve happiness?”

Human nature and love

This leads to another reason why nature matters when it comes to evaluating human sexual behavior. To reject human nature is to reject love. How so?

Recall that nature determines what is objectively good for a human being (see linked article above). Love, according to St. Thomas Aquinas, is to will the good of another (ST I-II:26:4). So, if we reject human nature, we reject what is good for the other. But if we reject what is good for the other, we fail to love.

Sexual behavior divorced from human nature undermines the very thing it’s supposed to express: namely, love. I guess Tina Turner’s song “What’s Love Got to Do With It?” applies after all.

Human nature and other types of behavior

A third reason is that we appeal to human nature to evaluate other types of human behavior. For example, we appeal to human nature when we say slavery is wrong. We recognize that slavery impedes the exercise of freedom that all human beings have by nature.

We also appeal to human nature when we judge murder to be wrong—it frustrates the intrinsic right to life that humans have by nature. Theft is considered a violation of our natural right to private ownership of goods and resources.

If we’re going to appeal to human nature to evaluate the morality of these non-sexual types of human behavior, then shouldn’t we appeal to human nature to evaluate the morality of sexual behavior?

Human nature and other types of sexual behavior

The appeal to human nature is justified also because as rational beings we appeal to human nature to evaluate some types of human sexual behavior.

Take for example bestiality, a topic I assume the majority of advocates for homosexual activity reject. When we say bestiality is not appropriate sexual behavior for humans, we are appealing to human nature. We recognize that such behavior is not conducive to what our sexuality orders us toward, namely another human person.

If we’re going to respect nature’s ordering of our sexual powers to another human person, then shouldn’t we also respect nature’s ordering of our sexual powers to a person of the opposite sex? Of course, nature’s ordering of our sexual powers to a person of the opposite sex is a point that has to be defended. But that goes beyond the scope of this article. Be on the lookout for a forthcoming blog post.

Another example is rape. We say rape is wrong because one is being forcedinto sexual activity contrary to his or her will. The assumption is that consent is necessary for appropriate human sexual behavior. But notice how such reasoning is premised on the idea that one should not physically impede another from exercising his or her freedom when it comes to sexual behavior. That’s an appeal to human nature. Even advocates of so-called “same-sex marriage” argue that consent is essential.

We also appeal to human nature when we object to adultery. Even though some may not be able to articulate the reason why adultery is wrong based on natural law theory (it threatens the stable union of husband and wife that is necessary for the rearing of children that sex brings forth), they intuitively recognize that sexual love is supposed to be exclusive. A person who lives the homosexual lifestyle typically is not going to be happy if his or her partner is sexually active with another person.

So since we appeal to human nature to judge otherforms of sexual behavior, it’s reasonable to appeal to human nature to judge homosexual behavior.

Human nature keeps morality objective

A fifth reason for not rejecting nature in our evaluation of sexual behavior is that moral evaluation of sexuality divorced from human nature becomes contingent on the subjective judgment of the individual, which in turn justifies any type of sexual conduct.

Let’s take the examples mentioned above. Consider bestiality. If sexual morality is relative to the individual’s will, then there would be no grounds for anyone to say bestiality is inappropriate human sexual behavior as long as the individual deems it appropriate.

Lest you think this is outside the boundaries of possibility, here’s what utilitarian moral philosopher Peter Singer has to say about it:

[S]ex with animals does not always involve cruelty. Who has not been at a social occasion disrupted by the household dog gripping the legs of a visitor and vigorously rubbing its penis against them? The host usually discourages such activities, but in private not everyone objects to being used by her or his dog in this way, and occasionally mutually satisfying activities may develop. Soyka [Viennese writer] would presumably have thought this within the range of human sexual variety (emphasis added).

After describing an incident where an orangutan seized a woman in response to his sexual instinct (intentions were made clear with the visibility of certain organs) at Camp Leakey, a rehabilitation center for captured orangutans in Borneo, Singer comments:

This does not make sex across the species barrier normal, or natural, whatever those much-misused words may mean, but it does imply that it ceases to be an offense to our status and dignity as human beings (ibid; emphasis added).

This is just one absurdity to which the divorce of moral evaluation of sexuality from human nature leads.

The approval of rape is another. If the use of our sexual powers is not governed by an appeal to what is good and bad for us given our human nature as spelled out in my article linked above, and is only based on what we feel, then on what grounds can we say the rapist is wrong? He may judge such activity is good for him and argue that he personally has a tendency to do these sorts of things.

Another example is “monogamish unions.”  A 2011 New York Times Magazine profile of Dan Savage, an American author and activist for the LGBT community, introduced Americans to the term “monogamish,” which refers to relationships in which partners allow sexual infidelity provided they are honest about it. In essence, this is a push to normalize adultery.

What if the couples both judge such behavior is appropriate human sexual behavior? Can we accuse them of being wrong? Not if sexual morality is divorced from the order of human nature.

The bottom line is that if moral evaluation of sexual behavior is divorced from nature’s ordering of our sexual powers, then sexual morality becomes relative to the will of the individual. And if sexual morality becomes relative to the will of the individual, then all types of sexual conduct can be justified, even the ones that we intuitively and rationally know are contrary to nature.

Human nature and intelligent use

Finally, the appeal to human nature and the ends toward which it orders our sexual powers is justified by the fact that we are rational beings. It belongs to our rational nature to ask, “What is sex for?” The late Frank Sheed, one of the greatest Catholic apologists of all times, comments:

I know that to the modern reader there seems something quaint and old-world in asking what a thing is for; the modern question is always, What can I do with it? Yet it remains a first principle of the intelligent use of anything to ask what the thing is for (Society and Sanity, 111).

In order to flesh this out a bit, consider a microphone. What if I thought the microphone was a hammer, and I used it to hammer some nails when building my house? Obviously I would destroy the microphone.

This illustrates the principle that in order to intelligently use something I must first know whatit is and what it’s for. If I use something contrary to its nature and what it’s meant for, I will likely destroy it.

The same holds true with our sexuality. We need to know what our sexuality is for before we can intelligently use it. But asking the question, “What sex is for?” is simply an appeal to the order that nature has inscribed within our sexual powers.

With regard to President Obama’s objection, how can asking what sex is for be an “outworn argument” and an “old attitude”? Must we stop asking what anything is for? If not, then why apply the principle only to sex?

It’s interesting that the very thing Obama thinks is not worthy of intelligent consideration, the appeal to the natural ordering of sex, constitutes a first principle of intelligent use for anything.


It’s unfortunate that Obama’s comments are similar to that of the character Skipper in the 2014 computer-animated movie Madagascar Penguins:“You know what? I reject nature.” If the appeal to that which constitutes us as a human being—namely, human nature—is an old attitude, then I guess the new attitude is the desire to be something less than human. I’ll take the “old” attitude. How about you?

This article is reprinted with permission from our friends at Catholic Answers.
Avatar photo


Karlo Broussard, a native of Crowley, Louisiana, left a promising musical career to devote himself full-time to the work of Catholic apologetics. For more than a decade he has traveled the country teaching apologetics, biblical studies, theology, and philosophy. Karlo has published articles on a variety of subjects in Catholic Answers Magazine, is a regular guest onCatholic Answers Live, and is an active blogger at catholic.com. Karlo holds undergraduate and graduate degrees in theology from Catholic Distance University and the Augustine Institute, and is currently working on his masters in philosophy with Holy Apostles College and Seminary. He also worked for several years in an apprenticeship with nationally known author and theologian Fr. Robert J. Spitzer at the Magis Center of Reason and Faith. Karlo is one of the most dynamic and gifted Catholic speakers on the circuit today, communicating with precision of thought, a genuine love for God, and an enthusiasm that inspires. Karlo resides in Murrieta, CA with his wife and four children. You can view Karlo's online videos at KarloBroussard.com. You can also book Karlo for a speaking event by contacting Catholic Answers at 619-387-7200.

Subscribe to CE
(It's free)

Go to Catholic Exchange homepage