Ketubah and why it's important from Gideon70's blog

Until the last couple hundred years, love marriages were not a thing.  People negotiated their marriages.  The reason for the change was the Monogamy Only doctrine.  Monogamy Only meant that to procreate, you had to have a single wife household.  If you had a child outside this stricture, you had a bastard which meant, roughly, "child outside of marriage of a nobleman."  If you married two women, the children were also considered bastards except the one by the first wife.  It was a way to guarantee lineage and transfer of property on death to punish anyone who dared violate the monogamy stricture.  The result?  Predictable.

Imagine you were married to a wonderful provider.  He died.  You were left alone.  Protections were assured for you, IF you lived in a poly world, but in a monogamy only world, you had a few choices.  You could give your children to a baby farm where they would be adopted out (or killed) and then you were free to attempt to find someone willing to marry you.  Your choices would be slim, and mostly undesirable.  You could attempt to work, and that left you at the mercy of your employer for his work and pleasure... because jobs were difficult to get, and you may get a good one where you worked on the floor in the daytime, and in his bed at night, or you were fired and left with the last and only option open to you.  Prostitution.

So there were very valid and legitimate reasons to make sure polygyny was banned.  Widows and divorced women were fun, easy to manipulate, and if they died because of illness or disease, no one really cared.

The Ketubah and plural marriage changed things for the better and would not be allowed.

A Ketubah was a marriage document.  It was negotiated by the woman/man and their families before marriage.  Romeo and Juliette was a cautionary tale of love marriages, they failed, and people died.  Arranged marriages were the standard, but we assume "arrange," meant that the parents arranged them, but that's not true.  Arranged meant the marriage was negotiated and the family was arranged by design.  They negotiated.

The new couple would carefully craft out their places in the marriage.  She was the head of the household, or the head of the farm, or the head of the kitchen or the wash... and the women had names you've heard.  The House Wife, The Farm Wife, the Washer Wife.  Each aspect of a household, laid out in areas of responsibility.  Just as the word for "Submit," means a woman submitting to a man as a Captain to a General, the Captain was expected to bring skills, talents, and leadership to one aspect of the Household or the entire household, although that would have been horrible.  Larger households would have been a nightmare to run by one person and probably one in a thousand could do it well.  So, the Ketubah.

The Ketubah was negotiated.  The family would sit down with close relatives (father, mother) and hash out what the authorities would be, and how they would apply.  She would generally be in charge of the household, and that could be as little as two room with a wash closet, or as large as an estate.  To get around the loss of other women to help, they would talk servants, paid help, and other employees.  She would run the house, he would run the business and often be the head of the Family, making the larger decisions.  If it were a plural family, servants would not be an issue, but a new potential wife would be asked to negotiate with the existing wife for her place in the family.  She might take over washing, or cooking, or childcare, splitting the duties and easing the burden on everyone. 

But the Ketubah was more than just that.  It was security.  It also covered what happens in a divorce, what happens to the kids, and offered (speculation) a power of authority for one wife to speak in his name in the event he was incapacitated to continue the family, or to dissolve it, with allotments to each of the women. It covered children, raising them, and expectations.  A woman knew her place, and had a place.  This is where the saying came from. If a woman got into the cake of another wife, she would be told, 'Woman, mind (or know) your place."  Marriages arranged like this had a terrible divorce rate.  It was almost 4%  Compared to love marriages where it's nearly 60% when people bother to marry.

It didn't leave women destitute, or at the mercy of unscrupulous men.