Ryan Anderson, co-author of What is Marriage? One Man, One Woman: A Defense writes for the Heritage Foundation's Foundry blog:

Some former officials in the Republican Party are urging the Supreme Court to redefine marriage for the nation. But support for marriage as the union of a man and a woman is essential to American—and conservative—principles. Indeed, nothing could be less conservative than urging an activist court to redefine an essential institution of civil society.

As my co-authors and I argue in our new book, What Is Marriage?, and in the amicus brief we filed with the Supreme Court, marriage exists to bring a man and a woman together as husband and wife to be father and mother to any children their union produces. It is based on the anthropological truth that men and women are different and complementary, on the biological fact that reproduction depends on a man and a woman, and on the social reality that children need a mother and a father. Marriage has public purposes that transcend its private purposes.

Marriage predates government. It is the fundamental building block of all human civilization. All Americans, especially conservatives, should respect this crucial institution of civil society. This is why 41 states, with good reason, affirm that marriage is between a man and a woman.

...Redefining marriage is a direct and demonstrated threat to religious freedom that marginalizes those who affirm marriage as the union of a man and a woman. We have already seen this in neighboring Canada and right here in places such as Massachusetts and Washington, D.C.

What should the Supreme Court do? The Supreme Court should not usurp democratic authority from citizens and their elected officials.