The answer to this question can be found in Chapter 6 of Mere Morality. Smedes suggests that the Seventh Commandment is not, contrary to Calvinist tradition, a "club against sexual passions." Rather, it is an injunction against violating covenants. Society, he says, tells us to be "self-maximizers." The Seventh Commandment tells us to be "covenant-keepers." The commandment "comes within a compelling call for fidelity." Smedes regards sexual intercourse as being a fundamental part of the marital relationship, and because it has such a binding and unifying effect, it cannot be shared outside the marriage while still maintaining the covenant intact. So a requirement to keep sex within a marriage is really a call to maintain an agreement that was established by God. Smedes does observe that the original meaning of adultery was understood as having sex with another man's wife. There was really no injunction against extramarital sex per se. But violating another person's marriage was a major blow at the covenants upon which society was based, so it was outlawed. Updated for modern sensibilities, however, the Seventh Commandment mandates that sex be kept within the context of a marriage, and adultery should be understood as the violation of the marriage covenant by either man or woman.