Why is fornication a sin, apart from God saying so?

What is the reason fornication is considered to be a sin apart from saying God said so? I am trying to find a secular and rational reason for this question.

It is reasonable to seek an explanation for why something is right outside of the commandment of God. But it is also unreasonable. The reasonable part has to do with how arguments are formed. If we want to demonstrate that some conclusion is correct (in this case, that non-marital sex is wrong), we rely on premises with which we have reason to believe our audience will agree. Then we argue that if the premises are true, the conclusion must follow. In that way, we show those who agree with the premises that they should, in fact, also agree with the conclusion. So, if we are trying to persuade a non-believer that sex outside of marriage is wrong, then we might think it important to have premises with which that non-believer will agree. Simply claiming the authority of God might seem like a non-starter in such an argument.

But the pursuit of an explanation detached from God is also unreasonable, for a couple of reasons.

Morality without God?

First, we should think long and hard about whether it is worth it to persuade a non-believer about some issue of morality without connecting that moral concern to God. To do so invites a step toward legalism or pharisaism — as if we are trying to fix people’s behaviour without them meeting Christ. Scripture makes clear that the message we bring is the good news about salvation, not about condemnation. The condemnation is already present, and even unbelievers already know it, whether they admit it or not — as John 3:17-18, Romans 1:18-20, and plenty of other passages make clear. So, convincing a person to behave morally without attaching that concern to faith in Jesus might actually do as much harm as good.

Convincing a person to behave morally without attaching that concern to faith in Jesus might actually do as much harm as good

Second, real morality has an inherent connection to a purpose transcending nature. Due to the limitations of this blog format, the case cannot be made here — but ethics or moralities seeking justification without appeal to something very much like God risk being reduced to utility: that is, “wrong” becomes nothing more than an abbreviation for “doesn’t work well in society”. Any such reduction is not actually about morality, but instead about functionality; and functionality (or utility) is no more right than preferring rice to potatoes.

All in the Golden Rule

Finally, though, there is the fortuitous reality that the same God who gives us moral edicts also gives us rationality as a tool for understanding Him and the world. So, there is a rational cause for concern about sexual immorality, and it is built into the Golden Rule — Matthew 7:12 — the reason all morality also makes sense. I first encountered the following concept in a work by Roger Scruton. There is no space to make the whole case, but here is the gist.

Sexuality is the most intimate expression of trust between people. Vulnerability and transparency are part of its nature. To receive another person’s total trust, but not give it to them at the same time, is to violate the very basis of morality — namely, that whatever is right or wrong for you is also right or wrong for anyone and everyone else.

Morality is nothing if not universal (a case for another day, but the point of both Matthew 7:12 from the scriptural side, and Immanuel Kant from the rational side). When one person gives total vulnerability to another, they should receive the same in return. That mutuality requires a unique commitment. “You can trust me with the innermost part of your soul forever, because I am committing the innermost part of my soul to you forever.” Anything less cheapens both the sexual experience itself and the value of the one participant to the other. Fornication and polygamy are excluded by exactly the same rationale.

Again, the format here precludes thorough demonstration, but the argument is there in its broad strokes. The bigger question is whether it is worth the time to make such a case about why someone should do right, without building that case on why they should know God. I suspect the better position is always to anchor our purposes and claims in the importance of and need for a relationship with God.