Same-Sex Marriage Will Harm Families

At the Heritage Foundation, "The Supreme Court’s Challenge: Restore Marriage Decisions to Citizens":
There are many good reasons why citizens in 41 states have said over and over that marriage is between a man and a woman. 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. And as ample social science has shown, children tend to do best when reared by their mother and father.

Government recognizes marriage because it is an institution that benefits the public good.

Marriage is society’s least restrictive means to ensure the well-being of future citizens. State recognition of marriage protects children by incentivizing adults to commit permanently and exclusively to each other and their children.

While respecting everyone’s liberty, government rightly recognizes, protects, and promotes marriage as the ideal institution for procreative love, childbearing, and childrearing.

In recent decades, marriage has been weakened by a revisionist view that sees marriage as primarily about emotional bonds or legal privileges. In other words, it is more about adults’ desires than children’s needs. Same-sex marriage is the culmination of this revisionism: Emotional intensity would be the only thing left to set marriage apart from other bonds.

Government should not obscure the truth about marriage by accepting that revisionist view. In redefining marriage to include same-sex relationships, government would weaken marital norms, which would further delink childbearing from marriage and hurt spouses and children—especially the most vulnerable. It would deny a mother or father to a child as a matter of policy.

The harms resulting from redefining marriage would force the state to intervene more often in family life and force the state’s welfare to grow even more. Citizens would lose more of their freedom of religion and conscience.
More at that top link.

And previously, "Same-Sex Marriage and Marxist Revolutionary Doctrine: Destroy the Family."