It's to make the point that if there is nothing special about a marriage being one man and one woman, then there is no rational basis for limiting marriage to only any two persons.
What's so darn special or sacred about the number two, once the traditional definition of marriage has been smashed?
Please explain how you can redefine marriage as two men or two women, and then hold the line there, and not go beyond the number two in forming the new definition of marriage.
If two men and a woman love each other and want to be united in marriage forever, would you forbid them that "right?"
Why? On what basis?