Why does the bible mainly focus on women committing adultery? Numbers chapter 5 describes a purity ceremony that determines if the wife cheated but there is no equivalent for men. It may be that the classification of adultery differs under a polygamous system.
It was adultery for a man to sleep with another man’s wife, but if a married man sleeps with an unmarried woman, it could count as fornication and he most likely will end up marrying her. Essentially a man could just marry a mistress as a second wife if he can afford it. A man (no distinction between married or unmarried is made in the text) who fornicates with an unmarried woman would pay the bride price, and if the father permits he would marry her (Ex 22:16-17), and he can’t divorce her (Deut 22:28-29).
This means if a polygamous man has sex with an unmarried woman it was fornication, not adultery. So while Abraham sleeping with Hagar in Genesis 16 was sinful because of his lack of faith and his sexual abuse of a slave, and there is no indication he married her, it was not technically adultery the way we think of it today. Meanwhile, King David had at least 8 wives (2 Sam 3:3-5, 2 Sam 5:13-16, 2 Sam 11:26-27) and at least 10 concubines (2 Sam 15:16, 2 Sam 5:13). However, it counted as adultery with Bathsheba because she was another man’s wife, and then he had the man killed and married her (2 Sam ch.11). Concubines are not wives yet they are allowed in a polygamous setting.
In Genesis 17, God makes it clear that all of Abraham’s children will be blessed (he promised a blessing for Ishmael in the previous chapter) but only the one that comes from Sarah (his first wife) will be the promised seed. Abraham has 8 children, one with Hagar, one from Sarah, and 6 more after Sarah dies with his new wife Keturah (Gen 25:1-5). God’s promised seed was Isaac (Sarah’s son) and indicated that with the name change of Sarah (Gen 17:15-22) and no one else. This shows that God favors monogamy and even Isaac is monogamous in his marriage. Notice Abraham never took another wife that whole time Sarah was alive and barren? We meet him at age 75 and he was monogamous that whole time. Hagar was just a one-time thing. Even with Abraham’s deceptive habits concerning his wife, God prevents the Pharaoh and King Abimelech from sleeping with Abraham’s wife (Genesis 12:10-20, Gen 20:1-18), God secures his covenant with Abraham to prevent him from screwing it up by actively protecting the sanctity of his marriage. This means God didn’t approve of Abraham giving away his wife, or his abuse of Hagar, yet he shows mercy and stays faithful to his promise.
Under a monogamy framework, it is adultery for a married person to sleep with anyone other than their spouse, and this is for both men and women. In most Abrahamic cultures, polygamy is limited to four wives since Jacob had only four wives but this limitation is not explicitly written in the Old Testament. Jacob stopped at four wives because his father in laws required it for their covenant (Gen 31:48-50). So under that principle, there could be a limit set by the fathers of one of the wives, and committing adultery would mean adding more wives since that is breaking the specific covenant set with the wives one has.
While polygamy was allowed in the Bible in the Old Testament, it has some restrictions. In the New Covenant, however, men are told to have one wife (1 Tim 3:2), mimicking Christ’s relationship with the one church (body of Christ). Polygamy was allowed in the Old Testament because there was only one group of covenant people (the Israelites) and their goal was to preserve until the Messiah came. In the old covenant, more covenant people are made by making more physical humans through reproduction because this covenant is built on blood relation to Abraham, and one way to ensure that your line carried on was to have more than one wife. However, there were grievous consequences and familial strife caused in these kinds of relationships since they are not the original model set up in Genesis 2:24. This is why in the new covenant making more covenant people is done by sharing the gospel so that people are born-again in the spirit. Therefore marriage is not necessary or valid in fulfilling the great commission. For a Christian who was already in a polygamous marriage before getting saved, he can either add a woman he fornicated with to his harem or never have sex with her again. Otherwise, he is committing adultery if he is breaking his contract with his current wives. Learn more about Biblical Polygamy here.