The Real Slippery Slope: Logic, Fallacy, and American Decline

Logic warns the wary of many fallacies. Everyone should master this canon of error—or you will be mastered by it. A logical fallacy is a typical way in which arguments go wrong through sloppy reasoning. Let me cite a few examples before turning to issue at hand—the slippery slope from same-sex marriage to significant changes in sexual ethics.

False dichotomy is a common and cunning fallacy. An either/or condition is set up. One must affirm either A or B, not both A and B, and not neither A or B. A proper dichotomy works like this:

You are either with Christ or against him. This covers all the available options. The condition is exclusive: Christ or not Christ (Matthew 12:30; John 14:1-6).

Here, though, is a false dichotomy:

We should not defend the gospel; we must preach the Gospel.

This is a false dichotomy because the Bible calls us to both defend gospel and to preach the gospel (Matthew 28:18-20; 1 Peter 3:15-16).

Another popular fallacy is argumentum ad hominem, or argument against the man. Instead of critiquing someone’s argument for traditional marriage, an opponent instead attacks the traditionalist’s character and motives. Some claim that denying same-sex marriage means one hates homosexuals and lesbians. This response gives us two fallacies for the price of one.

  1. The person’s character is attacked, rather than the force of his argument. This is a glaring case of argumentum ad hominem. Even if the critic hates homosexuals and lesbians (and no one should), that does not, of necessity, undermine his argument, since his argument may work independently of his motives.
  2. The false dichotomy is claiming that you either (A) support same-sex marriage or (B) hate same-sex couples.

I have introduced two classic and chronic fallacies. May we avoid them! Another fallacy feeds on itself. Meet the false charge of fallacy. Those pumped up by the power of logic may hyperventilate at the prospect of proving someone wrong through uncovering a lethal fallacy. Sometimes the difference between a fallacy and a valid form of reasoning is subtle. Consider the slippery slope.

As a fallacy, the slippery slope errs by wrongly stipulating the implications of ideas: if A, then B, then C. We always slip down the slope into the abyss. “If you do this, then that will happen, and, God help us, the other thing will happen, which is horrible!” So, the slippery slope also contains a reductio ad absurdum argument. Let me illustrate.

Some complementarians claim that if churches allow women to be senior pastors, they open the door to accepting homosexuality. Since most evangelicals—at least twenty years ago, when my wife was writing much on this topic—do not accept homosexuality, then they should reject women as senior pastors as well. But for evangelicals with a high view of biblical authority and a respect for classical methods of biblical interpretation, there is no danger of any slope to slide down, since it is obvious that while the Bible records many women leaders favorably, it never endorses any homosexual activity.

However, some ideas have consequences that may not be foreseen or that may be denied by sheer willfulness. Defenders of same-sex marriage often say that concerns over polyamory or incest are reactionary and those that are worried about this commit the slippery slope fallacy. However, the slope is there, the sliding is underway, but the slippage may be overlooked. Consider the Supreme Court decision on same-sex marriage.

According to the judges, no state may deny the right to same-sex marriage. The legal reasoning was tortuous and unsupportable, as the dissenting opinions stated. But we must go deeper. The idea behind the ruling—and behind all support for same-sex marriage—is that marriage is a socially constructed and purely human institution. Monogamy is merely a cultural and legal tradition whose cultural hegemony sputtered for years and is now ending. There is no reason why marriage should not be same-sex. In fact, to claim otherwise is to absolutize a relative and historically contingent institution. The same reasoning supported slavery, denied women the vote, and endorsed Jim Crow laws.

Given this standpoint, there is no reason to privilege couples only as constituting marriage. That would be two-ist and unfair. Therefore, by the same reasoning used to establish same-sex marriage, we must allow for polyamory. This is an open category which so far has meant marriage arrangements of at least men and women. (I have not yet heard of marriage arrangements of multiple men or multiple women.) A man has already sued to have two wives. A 2015 academic book by Ronald Den C. Otto is entitled In Defense of Plural Marriage. Amazon.com says this about it:

With over half of Americans now in favor of marriage equality, it is clear that societal norms of marriage are being quickly redefined. The growing belief that the state may not discriminate against gays and lesbians calls into question whether the state may limit other types of marital unions, including plural marriage. While much has been written about same-sex marriage, as of yet there has been no book-length legal treatment of unions among three or more individuals. The first major study on plural marriage and the law, In Defense of Plural Marriage begins to fill this lacuna in the scholarly literature. Ronald C. Den Otter shows how the constitutional arguments that support the option of plural marriage are stronger than those against. Ultimately, he proposes a new semi-contractual marital model that would provide legal recognition for a wide range of intimate relationships.

Once the traditional and God-ordained definition of marriage is breached, the floodwaters come rushing in. If love is love, and if any consensual erotic association should be deemed moral and be authorized by the state, then the logical implication is that the new sexual revolution will not end with same-sex marriage. The next step will be polyamorous marriages. Nor could there be a principle forbidding consensual incest or pedophilia. If you think pedophilia is different because the child is too young to give consent, you do not understand the deepest issues.

Breitbart.com posted a piece called “Inside the Sick, Secret World of Bestiality Forums.” The story is about BeastForum, a popular website trafficking in all manners of perversion. I will give no details. But if the animal is not hurt, and the human is not mean, why not practice bestiality? Given present trends, bestiality could emerge from the dark and dirty underside of American culture and move into the spotlight of our polymorphic perversities. (The Mosaic Law made bestiality a capital crime.)

Some will slide down this slope without fear, perhaps stopping shy of bestiality or a step before. Or perhaps not. But if one wants to apply the moral brakes, he or she needs a rational justification for stopping. I am not saying that most who deny traditional marriage will slide all the way down; but without an objective moral authority to tell us the nature of sexuality and to give the norms for sexual behavior, there are no such moral brakes.

However, if one who accepts same-sex marriage also opposes polyamory, incest, pedophilia, or bestiality, that person must face the full force of this reductio ad absurdum argument:

  1. If same-sex marriage is moral, then any consensual sexual arrangement (involving marriage or not) is moral.
  2. Same-sex marriage is moral.
  3. Therefore: consensual polyamory, incest, pedophilia, and bestiality are moral.
  4. But (3) is absurd, since these acts are immoral.
  5. Therefore, it is false that same-sex marriage is moral; it is immoral. This is by reductio ad absurdum.

One may not like this argument, but if the premises are true, then the conclusion must follow. I leave it to my readers to show that one or more of the premises are false.

When traditional moral authority is razed, then the unthinkable becomes thinkable, the illegal becomes legal, and the immoral becomes moral. This is the real slippery slope, and we are on it as a society.

May everyone—from politician to pauper, from professor to farmer, from all races, of all ages—hear the Word of the Lord:

Blessed is the nation whose God is the Lord,
the people he chose for his inheritance.
From heaven the Lord looks down
and sees all mankind;
from his dwelling place he watches
all who live on earth—
he who forms the hearts of all,
who considers everything they do.
No king is saved by the size of his army;
no warrior escapes by his great strength.
A horse is a vain hope for deliverance;
despite all its great strength it cannot save.
But the eyes of the Lord are on those who fear him,
on those whose hope is in his unfailing love,
to deliver them from death
and keep them alive in famine. (Psalm 33:12-19).

Author: Douglas Groothuis

Author of Christian Apologetics, Truth Decay, On Jesus, On Pascal, and others. Professor of Philosophy, Denver Seminary since 1993. Head of The Apologetics and Ethics Masters Degree Program and Co-Director of The Gordon Lewis Center for Christian Thought and Culture. Senior Fellow for Apologetics.com.

6 thoughts

  1. If you could please elaborate on the ‘deepest issues’ not understood by those of use who consider consent a large part of the ‘consensual sexual relationship’ as outlined in premise #1 of your argument I would be very appreciative.

  2. Dr. Groothuis,

    Thank you for posting the reductio ad absurdum. And thanks for your work on the blog.
    I agree with the conclusion of your RAA. But is the argument sound? Here is the argument as presented:

    1. If same-sex marriage is moral, then any consensual sexual arrangement (involving marriage or not) is moral.
    2. Same-sex marriage is moral.
    3. Therefore: consensual polyamory, incest, pedophilia, and bestiality are moral.
    4. But (3) is absurd, since these acts are immoral.
    5. Therefore, it is false that same-sex marriage is moral; it is immoral. This is by reductio ad absurdum.

    The argument is deductively valid. But is there is an equivocation on ‘moral’? In (1) and (2), ‘moral’ could seem to refer to non-objective (i.e., culturally relative or individually relative/subjective) morality, to morality that is a mere human construct. But in (4), ‘immoral’ seems to refer to objective morality. (4) seems to state that the acts noted in (3) are objectively morally wrong. If the meaning of ‘moral’ switches in the argument, then there is an equivocation and the argument is unsound.

    If there is no equivocation, then ‘moral’ is univocal in the argument. If so, is ‘moral’ univocal in the objective sense or in the non-objective sense? If the latter, the RAA would seem to be weakened, since the absurdity of (3) would be only non-objectively absurd, because the acts wouldn’t be objectively immoral. (Of course, they wouldn’t be objectively moral, either.)

    But your meaning appears to be the former. If so, perhaps it would be beneficial to explicitly note the objective sense.

  3. Suppose ‘moral’ univocally means “objectively moral.” An expanded version of the argument might look something like this:

    1. There is objective morality. (Premise)
    2. If there is objective morality, then an act can be objectively morally wrong. (Premise)
    3. Thus, an act can be objectively morally wrong. (1,2 MP)
    4. Same-sex marriage is objectively morally permissible. (Suppose for RAA)
    5. If marriage is a mere human construct, then marriage does not have an objectively real nature and vice versa. (Premise)
    6. Thus, if marriage is a mere human construct, then marriage does not have an objectively real nature. (5, SIMP)
    7. If same-sex marriage is objectively permissible, then marriage is a mere human construct. (Premise)
    8. Thus, marriage is a mere human construct. (4,7 MP)
    9. Thus, marriage does not have an objectively real nature. (6,8 MP)
    10. If marriage does not have an objectively real nature, then human beings can construct whatever definition of marriage they desire as long as the marriage includes proper consent. (Premise)
    11. Thus, human beings can construct whatever definition of marriage they desire as long as the marriage includes proper consent. (9,10 MP)
    12. If (11), then any merely human constructed sexual arrangement is an objectively permissible definition of marriage as long as the arrangement is consented to by those involved who are able to provide consent. (Premise)
    13. Thus, any merely human constructed sexual arrangement is an objectively permissible definition of marriage as long as the arrangement is consented to by those involved who are able to provide consent. (11,12 MP)
    14. If (13), then polyamory, incest, bestiality, and pedophilia are objectively permissible types of marriage as long as they are consented to by those involved who are able to provide consent. (Premise)
    15. Thus, polyamory, incest, bestiality and pedophilia are objectively permissible types of marriage as long as they are consented to by those involved who are able to provide consent. (13,14 MP)
    16. But (15) is absurd, since these acts are objectively impermissible (i.e., wrong) regardless of consent. (Premise)
    17. Thus, on the assumption of (4), these acts are objectively permissible and these acts are objectively impermissible. (15, 16 CONJ)
    18. Thus, it is false that same-sex marriage is objectively morally permissible. (by RAA, 4-17)
    19. If it is false that same-sex marriage is objectively morally permissible, then same-sex marriage is objectively morally wrong. (Premise)
    20. Thus, same-sex marriage is objectively morally wrong. (18,19 MP)

    * By an act being permissible, I mean there is no objective moral obligation to not do the act. In other words, PEp ↔ ~OB~p (where PE means permissible, and OB means obligatory). I.e., if action p is permissible, then there is no obligation to avoid p and vice versa. And if there is an obligation to avoid p, then p is impermissible.

    ** The argument rests on the assumption that if marriage has an objective nature (i.e., a set of essential properties), then that nature precludes same-sex union. In other words, the traditional definition of marriage aligns with the objective nature of marriage, but same-sex union does not align.

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