War (and privilege)
In this society, women are the leaders, generals, strategists. Men are the front-line troops. Leaders are valuable; grunts are expendable. If a queen has the forces to win a battle decisively, well and good. But if it takes sacrificing large numbers of troops to win, she will do what is necessary.
"I poured out the blood of my armies like water." - Empress Jadis of Charn
In a warlike society with this attitude, male mortality will be higher than female mortality, leading to a population with more women than men.
Now, each woman would prefer to have her own husband, or perhaps multiple husbands. But the population imbalance makes it impossible. The nobility use their power and privilege to marry as many men as they please. Everybody else has to settle for what they can get.
In this society, the family name is passed on through the mother. As in many real-world societies, people attach great importance to keeping their family name alive through their offspring. Most women, therefore, place a high priority on bearing children, and to bear children, you need a man. Sharing a husband isn't ideal, but it's better than remaining unmarried and childless.
(It is possible to have a child without being married, but women who go this route are typically looked down upon: it implies that they are so poor they couldn't even afford to share a husband.)