Paul is sending this letter to Timothy and he may have been in Ephesus. Marital customs of the time period are not as we know them today. The Jews had no mandate on polygamy and it continues on for several centuries after Christ. The Pagans (Gentiles) had several different arrangements and there is no clear understanding as to what happened to those arrangements if they converted to Christianity. The Romans considered the Jewish practice of polygamy to be heathenistic, "from their perspective" where their own marriages were not exclusive either. They only had one wife, but the men could have several sexual relationships, both men and women. Now it is hard to say what this scripture is actually addressing, Timothy is probably working with Gentiles. But to put it in a different context; If I have a company and I have a policy that in order to be in management you had to have a college education.... that does not mean that everybody in the company has a college education. This scenario also applies to the conversation where Christ is discussing divorce, He cannot be talking to Christians, because there is no such thing yet and it appears He is talking to Jews. Is he talking about someone that has one wife or five? If you divorce your wife, you cannot marry another. And then He references Mosaic Law which was polygamous, and then references what God said in Genesis. In Mark 10:12 He says,...and if she herself divorces her husband and marries another man, she is committing adultery. Which adds to the confusion because Judaism / Mosaic Law, had no provision for a wife to divorce her husband and the Christian church did not recognize a woman's right to divorce her husband, it was not legal until 1857. Christ mentions that Moses allowed divorce because of the hardness of their hearts...which also adds to the confusion because that statement sets the whole Mosaic Law on its ear. Either way, He completely flabbergasted His own Apostles. So there are some questions about all of that. Don't get me wrong, I do not believe Christians should be polygamous, but in that era several types of marriage arrangements existed and it would stand to reason that there was a transitional period.
Matthew 5:31-32 Mark 10:1-12