Sighs. Rolls eyes. Marriage has been a concern of government for literally thousands of years. Marriage has seen its ups and downs during that time, but the difficulties are rarely (if ever) caused by the government favoring marriage; rather, it comes when government denigrates its value.
So, how do you keep this particular government from denigrating it's value? And don't answer the FMA, because that will not happen.
How is this government going to manage to distribute all sorts of goodies to one segment of the population and deny it to another segment of the population? It just can't happen. Eventually government will define marriage down to the point where the institution of marriage is destroyed.
Social institutions like marriage should be preseved by social institutions like families, churches, and communities. Government can only destroy.