The short answer is this:
A free society requires formal and broad recognition that the natural family is the fundamental unit of society. Marriage is the cornerstone of the natural family.
Defining marriage as between a man and a woman is the central characteristic of this cornerstone because a man and a woman provide a free society with many vital benefits such as child-bearing and child-rearing.
An equally important benefit comes from the complementarity between a man and a woman – combined, men, women and children are healthier, more prosperous, better educated, happier, more communal and transcendent, and physically safer.
In other words, the natural family promotes limited government; any other “family” formation increases government dependency.
The recent Judge Shelby ruling, on the backs of several Justice Anthony Kennedy rulings, has dethroned the natural family as the fundamental unit of society and replaced the natural family with selfish individualism – meaning these radical court decisions seek to center a free society on any chosen behavior between consenting adults, regardless of the common good and the state interest.
If you believe in limited government within a free society, you would support the natural family as the fundamental unit of society.
For more on the social science research, you can read this report, “Why Marriage Matters,” found here: http://sutherlandinstitute.org/…/why_marriage_matters.pdf