The best response to proponents of gay marriage that I’ve ever heard, is the following segment from an article. “I oppose same-sex marriage (and no, I’m not a bigot), ABC News, (28 May 2015):
As we now understand it, marriage is not merely the expression of a love people have for each other. It is, or is intended as, a life-long union between two people who exemplify the biological duality of the human race, with the openness to welcoming children into the world. Even when children do not arrive, the differentiated twoness of marriage indicates its inherent structure.
Now, I didn’t pluck this definition from the sky, nor is it simply a piece of religious teaching. It is the meaning of marriage that emerges from all human cultures as they reflect on and experience what it is to be male and female. It is only in the last 15 years that anyone has seriously thought differently.
I prepare many couples for marriage each year. Most of them already cohabit. When I ask them about marriage, they almost always indicate that it is for them the beginning of a new family unit open to welcoming children.