In Judaism, adultery is considered one of the most grievous sins.
This article reviews the biblical and rabbinic teachings on adultery and its consequences. It should be noted that the Reform and Reconstructionist movements no longer apply the concept of mamzer (the offspring of an adulterous union), and that within the Conservative movement and Orthodoxy there is a strong tendency to avoid applying it wherever possible. The following is reprinted with permission from Every Person's Guide to Jewish Sexuality, published by Jason Aronson Publishers.
Adultery (sexual intercourse between a married woman and a man other than her husband [the biblical prohibition does not include sex between a married man and an unmarried woman]) is the only sexual offense recorded in the Ten Commandments. It is again recorded in the "Holiness Code" of Leviticus 20. The Book of Genesis (20:9) calls adultery "the great sin" and the Talmud calls adultery ha'averah (the sin par excellence). According to rabbinic tradition, it [along with incest, in the category of gilui arayot] is considered one of the three sins (along with idolatry and murder) that people should avoid even at the pain of death. The gravity of adultery is evident by the fact that the Bible describes the offense as being punishable by the death penalty for both the man and the woman.
One of the strangest of all biblical ordeals was that of a woman suspected of adultery (called a sotah in Hebrew). The procedure is described in graphic detail in the Book of Numbers 5:11‑31. Here the suspicions of a jealous husband may be proved or disproved by giving his wife a mixture of sacred water, earth from the floor of the Tabernacle, and the script of curses, and by observing the results of this ministration. If the woman had defiled herself by entering into an adulterous relationship with another man, the Bible states that her body would distend and she would become a curse among her people. But if the woman was not guilty, then she would remain unharmed and able to retain seed. Ordeals of jealousy were known in the ancient Near East, although not in the precise form described in the Book of Numbers, and there were parallels in many other cultures.
One thing made clear from this biblical ordeal of the suspected adulteress is that the Torah gives the male partner clear prerogatives by laying the burden of proving innocence on the woman. And, while both the wife and her adulterous lover were subject to capital punishment if guilty, no reverse ordeal was instituted: a wife suspecting her husband of infidelity had no recourse. The standards were not the same and men were allowed to be polygamous.
One major problem inherent in the law of the ordeal is the underlying assumption that by invoking the procedure a husband could force God, so to speak, to make the truth known. No other Torah law is dependent on such a divine manifestation.