Adultery is considered a sin in all religions.
The Bible decrees the death sentence for both the adulterer and the adulteress:
“If a man commits adultery with his neighbour's wife, both the adulterer and the adulteress shall be put to death.” (Lev. 20:10)
Islam also equally punishes both the adulterer and the adulteress:
“(As for) the fornicatress and the fornicator, flog each of them, (giving) a hundred stripes, and let not pity for them detain you in the matter of obedience to Allah, if you believe in Allah and the last day, and let a party of believers witness their chastisement.”(24:2)
However, the Qur’anic definition of adultery is very different from the Biblical definition. Adultery, according to the Qur’an, is the involvement of a married man or a married woman in an extramarital affair. The Bible only considers the extramarital affair of a married woman as adultery.
"If a man is found sleeping with another man's wife, both the man who slept with her and the woman must die. You must purge the evil from Israel" (Deuteronomy 22:22)
"If a man commits adultery with another man's wife both the adulterer and the adulteress must be put to death." (Leviticus 20:10)
“To keep you from your neighbour's wife, from the smooth tongue of the adulteress. Lust not in your heart after her beauty, let her not captivate you with her glance! For the price of a loose woman may be scarcely a loaf of bread, But if she is married, she is a trap for your precious life. Can a man take fire to his bosom, and his garments not burned? Or can a man walk on live coals, and his feet not be scorched? So with him who goes in to his neighbour's wife-- none who touches her shall go unpunished.” (Proverbs, 6:24-:29)
According to the Biblical definition, if a married man sleeps with an unmarried woman, this is not considered a crime at all. The married man who has extramarital affairs with unmarried women is not an adulterer and the unmarried women involved with him are not adulteresses. The crime of adultery is committed only when a man, whether married or single, sleeps with a married woman. In this case the man is considered adulterer, even if he is not married, and the woman is considered adulteress. In short, adultery is any illicit sexual intercourse involving a married woman. The extramarital affair of a married man is not per se a crime in the Bible.
Why is the dual moral standard? According to Encyclopaedia Judaica, the wife was considered to be the husband's possession and adultery constituted a violation of the husband's exclusive right to her; the wife as the husband's possession had no such right to him.1
That is, if a man had sexual intercourse with a married woman, he would be violating the property of another man and, thus, he should be punished. To the present day in Israel, if a married man indulges in an extramarital affair with an unmarried woman, his children by that woman are considered legitimate. But, if a married woman has an affair with another man, whether married or not married, her children by that man are not only illegitimate but they are considered bastards and are forbidden to marry any other Jews except converts and other bastards. This ban is handed down to the children's descendants for ten generations until the taint of adultery is presumably weakened.2
The Qur’an, on the other hand, never considers any woman to be the possession of any man. The Qur’an eloquently describes the relationship between the spouses by saying:
“And among His signs is that He created for you mates from among yourselves, that you may dwell in tranquillity with them and He has put love and mercy between your hearts: verily in that are signs for those who reflect." (30:21)
This is the Qur’anic conception of marriage: love, mercy, and tranquillity, not possession and double standards.