It's unfashionable and risks ridicule, and it's tough to maintain in the face of powerful cultural pressures.

But chastity remains a vital biblical virtue, argues Lauren F. Winner in her candid, commonsensical 21st-century manifesto, "Real Sex: The Naked Truth About Chastity" (Brazos Press).

Winner admits that she, like many young women today, indulged in premarital relations but gradually comprehended their sinfulness after the Christian conversion depicted in her winsome memoir "Girl Meets God" (2002).

She salutes new husband, Griff, "who had the foolhardy temerity" to date and then marry a woman who was writing a book about chastity.

Chastity involves everything from faithfulness in marriage to avoidance of pornography, but Winner's focus is premarital sex. (Chastity isn't the same as celibacy; abstention from sex is part of religious vocations and vows.)

Despite liberal Christians' attempts to define it away, Winner says limiting sex to marriage is an essential aspect of biblical teaching (but doesn't explore the same-sex marriage dispute). "You shall not commit adultery" is more explicit, but Winner believes the premarital ban is also assumed throughout scripture, beginning with Genesis and with Adam and Eve as sexual partners.

In 55 passages, the New Testament denounces "porneia," the Greek word for sexual immorality whose meaning varies in context. It's applied specifically to premarital sex in the Apostle Paul's admonition to unmarried Christians: "If they cannot exercise self-control, they should marry. For it is better to marry than to be aflame with passion" (1 Corinthians 7:9).

"If the only two options are marriage or smoldering with desire, it follows that sex outside of marriage is not an option," Winner reasons. "According to Paul, this sin is no minor peccadillo." Christianity was united on this until the "new morality" of the 1960s.

Dating do's and don'ts to maintain chastity are difficult to define. Lauren and Griff followed a rule suggested by a university chaplain in their town, Charlottesville, Va.: Do in private only what you're comfortable doing in public settings.

Winner says the culture spreads "four powerful lies" that accompanied the past half-century's moral individualism and promiscuity:

• "Sex can be wholly separated from procreation."

Yes, reliable birth control means we can usually disconnect sex from child-making, but this is only narrowly, "technologically" true, she says. Morally and religiously, sex has three purposes, to unite the couple in love, to produce children and to mirror "the radical fidelity and intimacy" between God and the church.

(Disagreeing with papal teaching, she believes "the whole of a married couple's sex life needs to be open to procreation" but not each sexual act.)