From Lauren F. Winner’s book, Real Sex: The Naked Truth About Chastity:
Let’s get down to brass tacks. What is chastity? One way of putting it is that chastity is doing sex in the Body of Christ—doing sex in a way that befits the Body of Christ, and that keeps you grounded, and bounded, in the community. As we’ve seen, that means sex only within marriage—which means, in turn, abstinence if you’re not married, and fidelity if you are.
Sex is, in Paul’s image, a joining of your body to someone else’s. In baptism, you have become Christ’s Body, and it is Christ’s Body that must give you permission to join His Body to another body. In the Christian grammar, we have no right to sex. The place where the church confers that privilege on you is the wedding; weddings grant us license to have sex with one person. Chastity, in other words, is a fact of gospel life. In the New Testament, sex beyond the boundaries of marriage—the boundaries of communally granted sanction of sex—is simply off limits. To have sex outside those bounds is to commit an offense against the Abstinence before marriage, and fidelity within marriage; any other kind of sex is embodied apostasy.
Chastity . . . is a spiritual discipline. Chastity is something you do, it is something you practice. It is not only a state—the state of being chaste—but a disciplined, active undertaking that we do as part of the Body. It is not the mere absence of sex but an active conforming of one’s body to the arc of the gospel.
From G. K. Chesterton’s essay, “A Piece of Chalk“:
Chastity does not mean abstention from sexual wrong; it means something flaming, like Joan of Arc. In a word, God paints in many colours; but he never paints so gorgeously, I had almost said so gaudily, as when He paints in white. In a sense our age has realised this fact, and expressed it in our sullen costume. For if it were really true that white was a blank and colourless thing, negative and non-committal, then white would be used instead of black and grey for the funereal dress of this pessimistic period. Which is not the case.