Marriage, concubinage, polygamy and having a mistress were originally all a little vague in their distinctions. Women, like dogs, camels, and sheep, etc., were a tradable commodity. The basic rule was “You shall not have more women, dogs, camels, and sheep, etc. than you can afford to keep.” This was enough to gain God's approval.
Although it is not mentioned in The NT, Josephus twice mentions concubinage as an institution and Justin The Martyr criticises Jewish teachers for the practice. It can be seen that, at the time of Christ, it was relatively common, regardless of adherence to a particular religion. This is not really surprising as OT characters often had many wives/concubines.
As Christianity progressed, the various NT statements that tend to support monogamy were increasingly read so as to prohibit polygamy (although none are beyond interpretations that permit polygamy.)
Tertullian wrote that monogamy was lawful, but that polygamy was not. Eusabius felt he ought to comment on the polygamy of the Patriarchs and this was then taken as some sort of approval (probably by those who had several wives or had ambitions in that direction.)
Basil of Caesarea wrote in the 4th century of plural marriage that "such a state is no longer called marriage but polygamy or, indeed, a moderate fornication." He ordered that those who are engaged in it should be excommunicated for up to five years, and "only after they have shown some fruitful repentance" were they to be allowed back into the church. Moreover, he stated that the teachings against plural marriage are "accepted as our usual practice, not from the canons but in conformity with our predecessors."
Augustine wrote in the second half of the 4th century: "That the good purpose of marriage, however, is better promoted by one husband with one wife, than by a husband with several wives, is shown plainly enough by the very first union of a married pair, which was made by the Divine Being Himself."
And so it was… well, at least for a time. The rule became strictly enforced after The Church held a synod in Hertford, England, in 673 that declared that marriage is allowed between one man and one woman, and separation (but not divorce) is only granted in the case of adultery, but even then remarriage is not allowed. (At this point I will mention that The Synod of Hertford was blissfully unaware of the Dominican Republic and seemed unaware of the messes that humans can get themselves into via their best intentions.)
Human reasonableness kept rearing its head and even Luther declared: "It is my earnest warning and counsel that Christians especially shall have no more than one wife, not only because it is a scandal, which a Christian should avoid most diligently, but also because there is no word of God here to show that God approves it in Christians.... I must oppose it, especially in Christians, unless there be need, as for instance if the wife be a leper, or be taken away from the husband in some other way."
Luther’s thoughts influence us all today, some more liberally than others, but forcing a doctrine upon people, or making judgements upon uncertain ground is never a good thing to do.