When I mentioned the Old Testament precedent regarding marriage, I refer to Matthew 19:5, where Jesus spoke of why a man leaves his father and mother and is joined to his wife. This is in reference to the first marriage, Adam and Eve, at the beginning of time.
 
This is the ideal held to in the traditional Western concept of marriage. I think these (and other Biblical) Scriptures have had a deep influence on Western thought, although there may have been some other influences as well. For instance, Greek culture I believe was monogamous.

One man and one woman joined for life is a pretty standard custom across many peoples and cultures. Though there are also other variations. I've lived in Africa for a while and have some friends who are polygamous. I knew one young man who married two young women in the same week.

But even in the Old Testament I think monogamy was the norm, with polygamy, though not specifically outlawed, being a bit unusual or sometimes adopted in unusual circumstances (e.g. a brother taking in his deceased brother's widow), and rarely if ever portrayed favourably. Kings had many wives for reasons such as assuring political alliances with foreign powers. Yet Jesus reaffirmed God's original intention for marriage pointing back to the first marriage in Eden..

My understanding of the revolutionary Family Law Act of the 1970s is that it was famous for setting up the Family Court and 'no fault' divorce, This was the government's rather imperfect way of dealing with marriage breakdown, which can be such a damaging and difficult thing that it ought to require some amount of regulation. However I don't think this Act attempted to redefine marriage away from the traditional concept, one man and one woman joined together for life.