Now that you put it that way, okay.
But why does that mean that the government should be involved in defining the meaning of a marriage contract?
We The People, through our representatives, enact laws to curb antisocial behavior. Leaving certain behaviors unregulated can sometimes be better, but too often invites the rule of vigilantes instead of the rule of law.
Here's an example. Left untouched by law, men are apt to abandon their women and children when the notion strikes them. Since law is required to thus regulate marriage, then a definition is required for what constitutes marriage.