Deuteronomy 19:15 One witness is not enough to convict anyone accused of any crime or offense they may have committed. A matter must be established by the testimony of two or three witnesses.
Deuteronomy 17:6
On the testimony of two or three witnesses a person is to be put to death, but no one is to be put to death on the testimony of only one witness.
Lev. 20:10 ‘The man who commits adultery with another man’s wife, he who commits adultery with his neighbor’s wife, the adulterer and the adulteress, shall surely be put to death.
Deut. 22:22-24 “If a man is found lying with a woman married to a husband, then both of them shall die—the man that lay with the woman, and the woman; so you shall put away the evil from Israel. If a young woman who is a virgin is betrothed to a husband, and a man finds her in the city and lies with her, then you shall bring them both out to the gate of that city, and you shall stone them to death with stones, the young woman because she did not cry out in the city, and the man because he humbled his neighbor’s wife; so you shall put away the evil from among you.”
The picture should be clear by now. They only brought the woman before Christ and were guilty of not fulfilling the Law of Moses by bringing the man as well.