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.
True. But if the vast majority of cops were crooked I would find hard to make a case for expanding the powers of law enforcement.
Government intervention in marriage is currently doing more harm to society than good. In fact, the State is using marriage as a excuse to dominate and control men and expand its powers over the upbringing of children.
Now you wish to Federalize marriage?
I could easily name 10 more urgent modifications to Constitution. This is feel good fantasy legislation. It won't do jack to prevent this society's slide into decadence.