Ephesians 5:31. Unambiguous. All other verses of that chapter in context show a marriage of one man and one woman, not limited to church leaders.
That’s aside from the example in Genesis 2:24, where God gave Adam one wife, not multiple wives. Noah also had one wife, as did his three sons have one wife each.
Also the injunction is made specially for kings, not to marry many wives, in Deuteronomy 17:17. (The later examples of David and Solomon, especially Solomon, show what happen when that’s ignored.)
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.