This is not a game with a zero-sum outcome.
We are talking about the rights of citizens, and the building blocks of family life.
If you take the position that same-sex marriage undermines opposite-sex marriage, you are casting the issue in zero-sum, terms.
The suggestion is:
To grant our homosexual citizens the same rights of any other citizen, the right to be married and to have their marriage contract honored wherever they go, to expand the franchise of citizenship in this way, encroaches on the rights of heterosexuals.
However, there is no such proposition. No one is saying that we should grant same-sex couples the right to marry, while restricting the rights of opposite-sex couples.
If these rights are rescinded by the courts, same-sex couples would be denied their actual rights, their standing in court, recourse to the law.
When the Supreme Court affirmed the rights of same-sex couples to be married, the opponents of same sex-marriage lost nothing, nothing substantial, nothing except a social (and legal) affirmation of their traditional values
This is what is good:
It is good to have our friends and neighbors, our brothers and sisters; and yes, even our mother and fathers, be who they are, living as they are, as fully enfranchised members of our society, without fear, or shame, or stigma.
When I was fifteen years old I had a girlfriend with a gay father.
She loved her dad and was proud of him, and though she did not shy away from the struggle of telling people her father was gay, it was nevertheless a struggle that she should not have had to face.
Her father took good care of her. Her father was in a steady relationship. If society could have recognized that relationship as legitimate, allowed him to marry his partner, made the issue of his sexuality into something that was not an issue at all, his burden would not have been her burden. In fact, his sexuality would not have been a burden to anyone at all.
It would have been good to spare them that angst.