Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body.
1 Corinthians 6:18

The English word "flee" occurs 81 times in the eminently physical contexts in the Old Testament—most of them having to do with escaping with one's life in the midst of conflict. In the more spiritually-focused New Testament, "flee" only occurs 14 times, but it loses none of its Old Testament urgency: run for your life; flee the enemy; escape while you can.

Therefore, when Paul told the Corinthians to "flee sexual immorality," he was being extremely serious. In the same way that we flee things that can hurt us, we should flee sexual immorality. Normally, we flee from an external force that might come against us. In the case of sexual immorality, we are fleeing the possibility of hurting ourselves. Who would want to harm themselves? Moreover, our body is not our own—it is a sanctuary for God's Holy Spirit. So not only do we sin against ourselves with sexual immorality, we sin against God within us.

Thankfully, God provides a way of escape from the temptation to sin (1 Corinthians 10:13). When you are tempted, look for the way He provides to flee.

Holiness is not freedom from temptation, but power to overcome temptation.
G. Campbell Morgan