Adultery is "voluntary sexual intercourse between a married man and someone other than his wife or between a married woman and someone other than her husband." 
Almost every Christian believes that adultery is immoral.
Although the Bible condemns adultery (it being the seventh commandment), God himself allowed Bible characters to engage in what most would consider adultery (or polygamy depending on interpretation). Genesis 16:1–6 explains how Abraham's wife Sarah prostituted her servant so that he could have children.
- "Now Sarai Abram's wife bare him no children: and she had an handmaid, an Egyptian, whose name was Hagar. And Sarai said unto Abram, Behold now, the LORD hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sarai. And Sarai Abram's wife took Hagar her maid the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram to be his wife. And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes. And Sarai said unto Abram, My wrong be upon thee: I have given my maid into thy bosom; and when she saw that she had conceived, I was despised in her eyes: the LORD judge between me and thee. But Abram said unto Sarai, Behold, thy maid is in thine hand; do to her as it pleaseth thee. And when Sarai dealt hardly with her, she fled from her face."
Either Abraham committed adultery with Hagar, or he married Hagar ("and gave her to her husband Abram to be his wife"), which would be polygamous.
Perhaps one of the Bible's few consistencies is the condemnation of adultery.
Exodus 20:14 : "Thou shalt not commit adultery."
Deuteronomy 5:18 : "Neither shalt thou commit adultery."
The Bible routinely establishes that the Christian punishment for adultery is death for both people involved. Daughters of priests, however, should be set on fire.
Leviticus 20:10 : "And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbor's wife, the adulterer and the adulteress shall surely be put to death."
Leviticus 21:9 : "And the daughter of any priest, if she profane herself by playing the whore, she profaneth her father: she shall be burnt with fire."
Deuteronomy 22:22 : "If a man be found lying with a woman married to an husband, then they shall both of them die, both the man that lay with the woman, and the woman: so shalt thou put away evil from Israel."
Matthew 19:18 : "He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness,"
Mark 10:19 : "Thou knowest the commandments, Do not commit adultery, Do not kill, Do not steal, Do not bear false witness, Defraud not, Honour thy father and mother."
Luke 18:20 : "Thou knowest the commandments, Do not commit adultery, Do not kill, Do not steal, Do not bear false witness, Honour thy father and thy mother."
Romans 13:9 : "For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself."
James 2:11 : "For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law."
The New Testament widened the scope of adultery to include re-marriage after divorce.
Matthew 19:9 : "And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery."
Adultery by God with Mary
According to the New Testament God hatches a plan to have Mary impregnated with Jesus. Mary agrees to this plan despite being betrothed to Jospeh. Luke 1:30-38 This could arguably be a case of adultery.
"He was a hidden God, full of secrecy. Verily, he did not come by his son otherwise than by secret ways. At the door of his faith standeth adultery."
Alternatively, apologists argue that since God can create anything, he created a sample of Joseph's semen and used it to impregnate Mary.  However, the issue is not about the origin of the semen used but who impregnated Mary!
Since this issue is predicated on the existence of God, most non-theists do not consider it particularly important.
In terms of social behaviour, condemnation of adultery is generally uncontroversial and commonly considered an immoral act. However, heated argument often rises when the issue of criminalization is discussed. Because most arguments for the criminalization of adultery in America are based in religion, many atheists believe that the law should not concern adultery due to the principle of church–state separation. Fundamentalist Christians, on the other hand, generally believe that the government should criminalize adultery.
Arguments proposing that adultery is not immoral are usually based on the principle that the institution of marriage is inherently immoral (or at least unnecessary). It would then follow that any action that could possibly deface marriage is necessarily not immoral.