What does 1 Corinthians 6:18 mean?

In the Greco-Roman, idol-worshiping culture of Paul's day, sex of all kinds had been normalized for nearly everyone. That included prostitution, adultery, pedophilia, homosexuality, and so forth. Growing up in this environment, it's not surprising to think some of the Christians in Corinth had trouble seeing sex outside of marriage as a big deal. Paul has spent this chapter showing why avoiding sexual immorality is so essential for believers.

Now he gives them a strategy for dealing with it: run. He tells them to flee from sexual immorality. Run away like you are escaping from something that might harm you, because it will. Even if the culture despises you for it, it's better to escape from sexual sin than to be conquered by it (Genesis 39:7–12).

Paul shows that sexual immorality is different from other kinds of sin because it's a form of self-harm. We might commit other sins with our bodies, but sexual immorality unites us sinfully with another person. This happens on a deeply physical and spiritual level. We will experience the natural consequences of that sin at that deep level, as well.

It's important to note that Paul did not write that sexual immorality is the worst of all sins, as we sometimes conclude. Instead, he is combatting the casual attitude toward sexual sin carried by some Christians in hyper-sexualized cultures. In addition to hurting others, sexual immorality contributes to our own deep pain. It's no more or less a sin than any other, but human cultures tend to treat it more casually than other errors.