As a Christian believer and a student of the Bible, I can tell you that there are a great many things which are believed by modern Christians which are not actually found in the Bible, and monogamy as a law is definitely one of them. Not only are there repeated instances of polygamy in the Bible which are never spoken against or punished by God, and are done by such heavyweights as Abraham, Jacob, David, and Solomon, among others, but God actually tells David that He was the one who gave David multiple wives and would have been happy to give David additional wives if he had only asked. This is in 2 Samuel 12:7-8 when God was rebuking David for committing adultery with Bathsheba and murdering her husband to cover it up. Note that David's sin was not taking multiple wives; it was adultery with someone else's wife. Similarly, God rebuked Solomon, but not for having multiple wives; He rebuked him for taking non-Israeli wives because they led him into idolatry and God had previously forbade taking non-Israeli wives for exactly that reason. In order to gain favor, Abraham famously gave his wife - who was very attractive - away to the king every time he entered a foreign city. It is recorded twice (Genesis chapters 12 & 20), and while chapter 20 specifically states that the king did not touch her yet, the king in chapter 12 admits that he took her as wife and she was brought into his house, which seems to imply that they had relations. Furthermore, it says in 20:13 that this was Abraham's custom wherever they went, so it might have happened on other occasions, as well, that Abraham gave his wife to other men. God never rebukes him for this.
Polygamy is never even mentioned in the New Testament, but there are hints that it existed and was allowed among Christians but not the elders of the church. 1 Timothy 3:2 says that the overseer (elder) "must be...the husband of one wife." Why would Paul state that requirement if every believer had to only have one wife? I would compare it with 3:3 where Paul says that the overseer must not be "an excessive drinker." That implies that drinking alcohol was permitted amongst the believers, including elders, but the elder must be strict with himself to exercise moderation and not be excessive in his drinking. Same with intersexual relations: elders should partake in sexual relations just like the rest of the saints, but he should act in moderation and only have one wife, whereas some saints may have more. There is no reason to mention excessive drinking unless drinking is allowed, and there is no reason to mention only having one wife unless more wives are allowed to the rest of the congregation.
This same criteria is mentioned (again by Paul) in Titus 1:6. The overseer must have believing children, not be drunken, not quick-tempered, not someone who strikes people...and "the husband of one wife." Again, each of the characteristics are in contrast to situations which existed among other believers and - although they may be undesirable traits - did not get them kicked out of the congregation. Not every believer has children who also believe in Jesus. Some are quick tempered. Some drink too much. Some might even strike people. And some might have more than one wife. It may be the case that the apostle Paul's opinion was that polygamy was not ideal, but he never said so. Instead, he only required that those taking the lead in each church were monogamous with one wife. Paul may even have desired, in general, that the other believers followed suit to be monogamous, since he considered the elders to be patterns for the flock. But again, an opinion, an encouragement, and a pattern are not a requirement and should not be applied as such. For example, Paul clearly states his opinion that it would be better for people not to marry at all, like him (the pattern), so that they could focus all of their attention on the Lord Jesus, but he does not make this a requirement on the believers, stating instead that "it is better to marry than to burn." 1 Corinthians 7:8-9.
I hope this helps anyone who is inclined toward scripture.