The short answer is that the case against same-sex marriage has been tested against reality. The case has not passed its test.
Since 1997, same-sex marriage has evolved from talk to fact.
If people like me had been right, we should have seen the American family become radically more unstable over the subsequent decade and a half.
Instead — while American family stability has continued to deteriorate — it has deteriorated much more slowly than it did in the 1970s and 1980s before same-sex marriage was ever seriously thought of.