Nations that fail to form families suffer grave consequences, including loss of workers, loss of tax base, and decline in human ingenuity and productivity.
The state has a vested interest in marriage and family because it is the bedrock of the nation. Same-sex pairs cannot benefit the whole of the nation by bearing the next generation. The population plunges (Demographic Winter), and all the attendant ills come roaring onto the political and social landscape. Those same ills have begun to reach other countries in the West: no families, no citizens, no economy, no national future. Many states have elevated marriage as a particular category within the culture to keep this vital institution as healthy as possible for the good of all citizens.