I understand that many claim for "sin" because something offend their sensibilities, but I think you take it too far. Ephesians is talking about a relationship, but doesn't condemn polygamy. Adam being given but one wife is lame. Deuteronomy also says kings shouldn't amass too many horses or too much silver - the intent of the passage you cite is obviously telling kings to stay real and grounded lest they become so disconnected that they cannot be effective/just rulers.
I don't understand why folks want to marry more than one other mate. I don't count the mere act as sinful - the reason it is done may make it sinful, but not across-the-board.
I'm far more concerned with the homosexual agenda's destruction of family values. I won't fight it, but I also won't defend it. It is what it is and it will only get worse and will not go away. If you believe in the Bible, and that God has already ordained that the world will become a moral cesspool before Jesus comes back to set things right, then you should also know that we can refrain from indulging, but we cannot stop what is happening. He will have His way and we can only tend to ourselves and those we have relationships with.
In prescribing monogamy, there is the condemnation of polygamy. Unless Mohammed’s habits in this vein are not worthy of condemnation?
The gays may attack family values of their own volition and their own agenda, but the no-fault-divorce crowd and polygamists attacked them first.
It’s not implied that we can stop what is happening—even the Bible says we don’t have the power to stop it (2 Timothy 3:13), but there is one who will.
It's in the LARGE handbasket.
How do I get my OLD country back?
2 Chronicles 7:14
If my people, who are called by my name,
shall humble themselves,
and pray, and seek my face,
and turn from their wicked ways; ,
then will I hear from heaven,
and will forgive their sin,
and will heal their land.