The institution of marriage predates government and has served as the foundation of society for thousands of years. Marriage is also the foundation of the family.
Why has virtually every society throughout history defined marriage as the union of one man and one woman? The answer can be summarized in one word: children.
Protecting the interests of children is the primary reason that government regulates and licenses marriage in the first instance.
Marriage between a man and a woman protects and promotes the well-being of children by allowing the child produced from the sexual union of the two adults to benefit from being raised by both her father and a mother.
While death and divorce too often prevent it, children do best when raised by a loving mother and father within the bounds of marriage. Every child deserves to be loved by her mother and her father.
Marriage is a special relationship reserved exclusively for heterosexual unions because only the intimate relationship between men and women has the ability to produce children as a result of that sexual union.