[...a constitutional amendment defining marriage would be a radical intrusion on the nation's founding commitment to federalism in an area traditionally reserved for state regulation, family law]
This is absolutely correct.
The constitution is supposed to define what the government does and does not do. It is not supposed to limit the behavior of citizens. Like abortion policy, state law is the only proper place for this.
And what part of, no state can force, through judicial fiat, homosexual marriage on it's citizens, is not "define what the government does and does not do. "