You wrote: "In the NT, where Christianity is moving into Pagan cultures where polygamy may be the norm, the guideline for Christian behavior is being laid out."
Let me stop you right here. To be more precise, prior to Jesus's resurrection, a covenant relationship with God was only available, more or less, to the descendants of Abraham. After Jesus died and rose EVERYBODY could have that same if not better covenantal relationship with God.
So, to characterize this phenomena as 'Christianity moving into pagan cultures' while not untrue, reflects a rather fuzzy kind of thinking. But, your point is well taken and SPECIFICALLY addressed in the Bible. The Jewish leaders came together and, seeing that gentiles were now allowed to have covenant with God (which at first blew them away) made decisions on how they should behave. Their decision is found in Acts 15:19-20:
19 "It is my judgment, therefore, that we should not make it difficult for the Gentiles who are turning to God. 20 Instead we should write to them, telling them to abstain from food polluted by idols, from sexual immorality, from the meat of strangled animals and from blood.
If you're line of reasoning were correct, than they most certainly would have told these Pagan cultures where polygamy may be the norm to stop having more than one wife. But they didn't. Sexual immorality basically means having sex with someone you're not married to. Like David did with Bathsheeba, for which he was condemned. He was NOT condmened for taking more than one wife however.VERY important and foretelling distinction.
"Granted this is for church leaders, but the principle of one wife/one husband for everyone is an underlying there."
Not really.
Mature Christians, the type who led churches, are to have one spouse. Elsewhere in the NT all are exhorted to grow into maturity. I see that as the point to refute those who don't aspire to become elders/deacons.
Now you are conflating 'maturity' with practical requirements made of church leaders. But the Bible makes it very CLEAR that only married men (and married one time only) are to lead churches. Does that mean women cannot be mature? Or single men? There's no such thing as a mature divorced man???