The story describes how sin enters the picture as the result of twisted desires. Adam and Eve fall prey to the serpent's suggestions about what they really want (the serpent is the mythological ancestor of advertisers), and their desires turn from each other to a false abstraction: being "like gods" (3:5). The perversion of desire is the paradigm for McDonaldized sex: one no longer desires a person and all the goods that accompany what it means to be a fully enfleshed human being—what the Psalmist describes as "little less than a god" (8:6)—and begins desiring some abstraction, like social capital or a sense of power. One is willing to substitute a small good (bodily pleasure) for an eternal good: the unfolding of divine love through the mediation of one's whole self in relationship to the other.

The McDonaldization of sex—the flattening of expectations in a sexual relationship—is not about desiring too much; it is about desiring too little. And with McDonaldized sex has come the near-absolute marginalization of a virtue that the ancients understood and that we mock: chastity. Very different from celibacy or abstinence from any kind of sexual expression, chastity is to sex what virtuosity is to music, or whatever we would call the ability in great athletes as compared to ordinary physical exercise. Chastity is a form of excellence in relationship, and involves all our faculties, all of who we are, including our sexuality.

Contraception obscures a profound dimension of ourselves: our ability to become "one flesh" with another. It is remarkable that our reproductive capacity is the only system in the human body that cannot function on its own; it must work in concert with the system of a person of the opposite sex. Obscuring that capacity in a relationship makes it very difficult to practice chastity, here defined as "excellence in sexual relationship," with a lifelong partner. And why is such a practice useful? Because chastity by definition must be lifelong, and over the course of life that practice becomes mandatory. Why? Because human beings get sick; they travel; they have children who interrupt their sex life; they get stressed at work; they experience depression; they have periods of loss and of grief. It is critical to the success of a lifelong relationship to develop chastity, including at times specifically abstaining from copulation altogether, simply because even if it is not practiced by choice, at times it will happen by virtue of necessity.

The alternative to contraception isn't asexuality, coldness in relationship, or complete abstinence from any form of sexual expression. It is the organic practice of chastity, excellence in all forms of relationship, including the sexual. That, I want to suggest, is why of all the metaphors for divine love in the Bible, marriage is used as the primary one to convey the way that God loves. For a marriage in which the partners commit to encouraging each other in the practice of chastity is one in which the priority is on sustaining love for the whole of life, which is as close as we get to mirroring God's love.