Matthew 5:28: Adultery in the Heart?
This is another instance of Jesus' making the law more stringent by carryingits application back from the outward act to the inward thought and desire. The seventh commandment says, "You shall not commit adultery" (Ex 20:14). In the cultural context of the original Decalogue, this commandment forbade a man to have sexual relations with someone else's wife. To infringe this commandment was a capital offense; the penalty was stoning to death (as it still is in some parts of the Near and Middle East). Another commandment seems to carry the prohibition back beyond the overt act: the second clause of the tenth commandment says, "You shall not covet your neighbor's wife" (Ex 20:17), where his wife is mentioned among several items of his property. In a property context one might covet someone else's wife not by way of a sexual urge but because of the social or financial advantages of being linked with her family.
However that may be, Jesus traces the adulterous act back to the lustful glance and thought, and says that it is there that the rot starts and it is there that the check must be immediately applied. Otherwise, if the thought is cherished or fed by fantasy, the commandment has already been broken. There may be significance in the fact that Jesus does not speak of someone else's wife but of "a woman" in general. Parallels to this saying can be found in rabbinical literature.
Pope John Paul II excited some comment in 1981 by saying that a man could commit adultery in this sense with his own wife. Emil Brunner, in fact, had said something to very much the same effect over forty years before. But there is nothing outrageous about such a suggestion. To treat any woman as a sex object, and not as a person in her own right, is sinful; all the more so when that woman is one's own wife.
See also comment on JOB 31:1.
Emil Brunner, The Divine Imperative (London: Lutterworth Press, 1937), p. 350.