The debate over marriage is centered on the misconception that government defines marriage. It does not. Marriage is defined by sexual complementarity, which predates any form of government. Government confers certain benefits on marriage that it does not currently provide to other familial arrangements which are based on love and commitment.
Once sexual complementarity is removed from the definition of marriage, then what's to prevent the creation of other loving, committed relationships, such as mother-daughter, father-son, brother-brother, sister-sister, from seeking government support and benefits for their family situation? Why would the government reject unions of more than two people? All rational reason for rejecting these situations goes away and opens up the question of unequal treatment under the law to be debated in the courts for years to come.
The desire of society to exercise their discretion in redefining marriage creates a host of unintended consequences. As such, if government decides that marriage no longer merits special treatment, it should focus on redefining it entitlement programs, tax code, property and inheritance laws so that all familial arrangements are treated equally under the law.