What is the whole moral principle of biblical teaching?
The whole moral principle of biblical teaching is that believers, being in union with Christ, must be as careful about their fidelity to Him as a husband or wife should be to each other. Sexual immorality adulterates that union, so a believer has no business involving himself in extramarital relationships.
Les explains that because believers are now in union with Christ, they are to be just as careful about their fidelity to Him as a husband or wife should be to each other. He identifies this as the whole moral principle of biblical teaching, citing I Corinthians 6. Paul is pressing the Corinthians: whatever their past lifestyle, they are now children of God on totally different territory and are totally different persons. If they involve themselves in immoral activity, they are literally adulterating their position with Christ. Scripture says every other sin is outside the body, but the one who commits fornication sins against his own body. Therefore the believer has no business in extramarital relationships, because the family and marriage relationship is holy, sanctified, and ordained by God. Les also notes he quit goose hunting because geese are monogamous.