Should a believer be involved in extramarital relationships?
No. Scripture teaches that a believer has no business involving himself in extramarital relationships, because he is now in union with Christ and must be as faithful to Him as spouses should be to each other. Committing fornication sins against one's own body and adulterates that position with Christ.
Les teaches that because the believer is now in union with Christ, he is to be just as careful about fidelity to Him as a husband or wife should be to each other. This is the moral principle of biblical teaching Paul brings out in I Corinthians 6: every sin a man commits is outside the body, but he who commits fornication sins against his own body. Les explains that Paul is pressing the Corinthians: no matter what their past lifestyle was, as children of God they are on totally different territory and are totally different people. If a believer involves himself in immoral activity, he is literally adulterating his position with Christ. Therefore, the believer has no business involving himself in extramarital relationships. Les also notes that the family and marriage relationship is holy, sanctified, and ordained by God, and he illustrates this by mentioning that he quit goose hunting long ago when he learned geese are monogamous.