Well of course it was defined that way in Genesis 3. That's all there was then--One man and One woman.
David was not condemned for his plural wives until after he lusted after Bathsheba.
You can read the Bible as a rule book or as an insight into the ways of G-d and His relationship with His people. It has elements of both.
David's plural wives were not condemned directly, although they were condemned in the passage about not multiplying wives. But it was clearly David's willingness to ingulge his appetites, including multiple wives, that eventually lead to his most egregious sin - the murder of Uriah to cover up David's adultery.
The concept is also made clear when Abraham tries to fulfill G-d's promise through Hagar rather than his wife Sarah. G-d does not bless Ishmael nor pass the promise through him. G-d blesses Isaac who is the product of Abraham's only legitimate wife.
Abraham had no reason to think he was wrong to father a child through Hagar. It was normal in Chaldean society for men to do this to have an heir. However, G-d's ways are not man's ways. G-d went to a lot of trouble to bring Abraham out of Chaldea to a place where G-d could teach Abraham what He wanted for us. And in the issue of marriage Ishmael and Isaac became the focal points of the lesson. G-d intends for a man to marry one woman and to father all his children through her.
Keep in mind that G-d also doesn't condemn divorce. He actually creates a law for it. But He also makes it clear that He hates it. While we are fallen He makes allowances for things that He doesn't intend. Our goal as the fallen is to try to give Him what he intends - not look for excuses to do things our own way.
Shalom.