Why does the Bible compare sex to fire?
The fire comparison illustrates that sex is a gift from God: under His mandated controls it is glorious and beneficial, like fire in a fireplace, but turned loose it brings devastation, ruined lives, ruined families, and ruined marriages. Misusing this God-given gift produces tragedy rather than blessing.
Les explains the comparison by pointing to fire under control: it warms a home, makes a glowing fireplace or wood heater on a cold day, and is necessary in a steel mill and everyday life; it is a blessing of God. But fire turned loose as a wildfire brings devastation, tragedy, and destruction. He says that is what sex is for the human race: under the controls God mandated, it is just as glorious as fire in the fireplace, but turned loose as the world has done, it becomes tragedy, ruined lives, families, and marriages. The preceding verse says fornication sins against one's own body, reinforcing that misappropriating God's gift of sex harms the person.