Advocates for one man, one woman marriages base their argument on Christian values and "God's plan" for marriage. An examination of Biblical passages regarding marriage, however, reveals this traditional model is not a Biblical concept.
Marriage is not about sex or love, it's about ownership, property.
In Biblical times. woman were viewed as property. A man who had reached a certain amount of status and wealth sought to marry/own a wife. Exodus 20:17: “You shall not covet your neighbor's house; you shall not covet your neighbor's wife, or his male servant, or his female servant, or his ox, or his donkey, or anything that is your neighbor's.” Wives were the property of their husbands, same as his servants and asses.
A woman's sexuality was also his property. Her sexuality belonged to her father until he sold/married her to her husband, or the highest bidder. Men were permitted to have sex with their slaves as this was also their property. The major difference between wives and slaves was the status of offspring. Legitimate male babies were in line to inherit their father's wealth. Legitimate female babies could be sold as a wife. Any illegitimate offspring would only demand the price of a slave, male or female.
Nowhere in the Bible does it say marriage is between one man and one woman.
Today, rich men collect expensive cars, large estates or private islands. The nomadic Patriarchs collected beautiful women to display their wealth. Men were permitted to have as may wives as they could afford. More wives meant a greater number of offspring, another symbol of wealth.
Women, however, could not have more than one husband, as she could only be owned by one man at a time. Having sex with a man who was not her husband was considered adultery and punishable by death. Think about that the next time you take your friend's expensive car out for a joy ride.
Even in the New Testament, while polygamous marriage was less common, women were still the property of their husbands. Permitted to speak to other men only in very limited circumstances.
There are no instances where Jesus or any of the New Testament writers specifically define marriage as one man with one woman.This is because it is assumed that the Law of Moses (read: Old Testament) statutes still apply. If Jesus wanted to deviate from the Jewish Law - which the Jews of his time followed literally - he would have spoken directly on the matter.
There is no such thing as "the way it's always been."
Monogamous marriage has only existed as the cultural norm for the last couple of centuries. The option of choosing one's marriage partner (at least for women) has only been realized in the last 200 years.
Views on societal norms are changing, have always been changing, and will continue to change long after you and I have vacated the planet. Only 200 years ago, slavery was viewed as "the way it has always been and always will be."
There is no final destination when it comes to defining human relationships. How we define them will change as we come to understand oursevles better individually and collectively.