"Chastity is the most unpopular of Christian virtues."

As Lewis goes on to say, "The Christian rule is, 'Either marriage, with complete faithfulness to your partner, or else total abstinence.'" Passages supporting this statement include the following, taken from the New International Version.

Exodus 20:14: You shall not commit adultery.

Proverbs 5:16-20: Should your springs overflow in the streets, your streams of water in the public squares? Let them be yours alone, never to be shared with strangers. May your fountain be blessed, and may you rejoice in the wife of your youth. A loving doe, a graceful deer--may her breasts satisfy you always, may you ever be captivated by her love. Why be captivated, my son, by an adulteress? Why embrace the bosom of another man's wife?

Matthew 5:28: But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.

Acts 15:20: Instead we should write to them, telling them to abstain from food polluted by idols, from sexual immorality, from the meat of strangled animals and from blood.

1 Corinthians 6:13, 15-19: "Food for the stomach and the stomach for food"--but God will destroy them both. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body. . . . Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh." But he who unites himself with the Lord is one with him in spirit. Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body. Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own.

Ephesians 5:3: But among you there must not be even a hint of sexual immorality,or of any kind of impurity, or of greed, because these are improper for God's holy people.

Colossians 3:5: Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry.