Family law is designed to protect families, for the betterment of society. So we have laws regarding child support, divorce, bigamy, etc.
Yeah, right. Family law currently turns men into second class citizens the moment they are married and indentured servants the moment they have children (or if their wives have the child of another man).
Spare me the "betterment of society" crap. Modern family law is for the betterment of the divorce lawyers, the feminization of society via activist judges, and the enslavement of men.
Whatever family law is in practice doesn't negate its legitimacy in principle, just as the existence of crooked cops doesn't negate the legitimacy of law enforcement in principle.