Maggie Gallagher was defending traditional marriage on a blog. I think that she is correct that traditional marriage is deteriorating, and that support for same-sex marriage is a factor, but there are several bigger factors. Here are a few.
Feminism. American women are treated better than any class of people in the history of civilization. Feminists somehow think that women are being mistreated, and they are not happy in a traditional marriage.
Decoupling of marriage from custody and support. Marriage once meant an agreement to jointly share the rights and responsibilities for any children that result. Now family courts usually completely ignore whether there was a marriage or not when they consider child custody and support.
Monetization of fatherhood. Father's rights have been attacked to the point where fathers are valued only for what they pay in child support. Mothers have a big incentive to get a divorce, take the kids, and collect child support.
Undermining parental rights. Various liberal ideas, such as "it takes a village to raise a child" and "the best interest of the child" have been used to undermine parental rights, and give teachers, physicians, judges, and others authority over children. Part of the purpose of marriage is to take responsibility for children, and when some of that responsibility is removed, marriage has less purpose.
No-fault divorce. Anyone can walk now out of any marriage for any reason, and it is usually the women who walk. Divorce court litigation is nastier than ever, for other reasons, but marriage has been effectively redefined to mean that spouses have very few marital obligations to each other.
Domestic violence enforcement. Domestic squabbles can now result in arrest and prosecution, even if no victim wants to press charges. Laws like VAWA work to bust up marriages by promoting discontent. There used to be a marital privilege protecting spouses from testifying against each other, but it is now ignored.
Public approval for adultery and other sexual arrangements.
Same-sex marriage. The homosexual lobby tries to gain social acceptable by redefining marriage. The new definition is a departure from traditional purposes of protecting children.
Sperm donation and other reproductive technologies. It is now legal for a woman to have a child with no legal father.