I see. Well, while there may be other benefits to society in recognizing it, the core reason is about children, whether or not all heterosexual unions produce children. It is irrelevant that every heterosexual union does not produce children. The important thing is that every child comes from these unions, AND ONLY THESE UNIONS. Minus mom and dad, children become the responsibility of government. That is HUGH AND SERIES.
Male polygamy and female hypergamy, unregulated, will destroy any society.
The man-woman relationship is also an important template for the children, regardless of whether the mother and father are adoptive or biological.