There are two types of adultery spoken of in the Bible:
SEXUAL ADULTERY -- "Conjugal infidelity. An adulterer was a man who had sexual intercourse with a woman he was not married to, either a married woman or one who was engaged to be married. A woman who did the same thing was an adulteress. Adultery is fornication. The Bible regards adultery as a great sin and a great social wrong. It has been inferred from John 8:1-11 that this sin became very common during the age preceding the destruction of Jerusalem."
The Mosaic law (Num. 5:11-31) prescribed that the suspected wife should be tried by the ordeal of the "water of jealousy." There is, however, no recorded instance of the application of this law. In subsequent times, the Rabbis made various regulations for discovering the guilty party, and of bringing about a divorce.
Adultery without sexual intercourse -- Jesus Christ warned, "You have heard that it was said to those of old, 'You shall not commit adultery.' But I say to you that whoever looks at a woman to lust after her has committed adultery already with her in his heart." As with the breaking of any of the Ten Commandments, those who are not washed in the blood of Jesus Christ will have to pay the penalty. They will not enter the Kingdom of God. Punishment for transgression of this Commandment is the death penalty.