You cannot protect marriage by denying marriage to same-sex couples. That should be clear from the basics.
For centuries, marriage as a legal, civil and religious institution between a man and a woman has protected children and society in every country and culture.
Granted for the sake of argument, but why is that being put forward as a reason why marriage as a legal, civil and religious institution between two men or two women can’t be allowed to protect children and society in the US? Are these people so busy “protecting marriage” they’ve lost track of what marriage is all about? Evidently. How sad.