God’s Law specifies that man cannot lie with man, that a person cannot have sexual relations with someone very close in kin, and that adultery is a grave sin. However, there is no place in His Law where it says that a man or woman cannot have more than one wife or husband.
Genesis 2 states marriage clearly thus:
And the Lord God said, “It is not good that man should be alone; I will make him a helper comparable to him.” Out of the ground the Lord God formed every beast of the field and every bird of the air, and brought them to Adam to see what he would call them. And whatever Adam called each living creature, that was its name. So Adam gave names to all cattle, to the birds of the air, and to every beast of the field. But for Adam there was not found a helper comparable to him. And the Lord God caused a deep sleep to fall on Adam, and he slept; and He took one of his ribs, and closed up the flesh in its place. Then the rib which the Lord God had taken from man He made into a woman, and He brought her to the man.
And Adam said:
“This is now bone of my bones
And flesh of my flesh;
She shall be called Woman,
Because she was taken out of Man.”
Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh. And they were both naked, the man and his wife, and were not ashamed. ~Genesis 2:18-25
The above is usually understood to mean one man and one woman, but the text does not specifically refer to ONE man or ONE woman.
Having said that, most, if not all, of the major sons of God, the Patriarchs and Prophets, married only one wife. And the ones who married more than one wife at the same time, Abram and Jacob, had stories and lives that were less than ideal because of it. It can cause great infighting among the children born of the different wives. Moses had two wives, but not at the same time. Adam and Noah had only one wife. Jesus had no wife.
So, while God illustrates in our Biblical history that monogamy is preferred over polygamy, He does not expressly forbid multiple marriage. God understood that there were times when men might be at a shortage and, to properly be taken care of, women would have to share one man. The first commandment of God was to go forth and multiply. If there is a paucity of men, and only one man can marry one woman, that would not be possible. Furthermore, the way God made fertility allows for a male to easily create as many children as his wives can have, increasing God’s chosen ones. One woman for many men would limit the offspring to only as many as one woman could have.
There is no law stating that two women cannot have sexual relations with each other. God understands that a man might want to be with his wives at the same time! That is a natural human urge, and, when it occurs within the scope of marriage, is not a sin, but a pleasure to be enjoyed from God.
It is Christianity that bestowed monogamous ideals, largely through cultural and social domains; those with power profited by society being monogamous. Jesus did not speak against marriage, only divorce and adultery. Adultery is any type of sexual immorality outside of marriage. Though the push for monogamy can be attributed to Christian ideals, it was not necessarily Christ’s teaching.
God allows for polygamy, which is usually manifested as polygyny, historically and culturally. The scriptures illustrate that polygamy is not ideal, monogamy is preferred, and celibacy is the highest state of living, if one can attain it. Yet God allows us to find our own comfort zone, ideally led by Him, in this wide range of models for romantic loving relationships.