Exactly what part of the Constitution does it violate?
The US Constitution says NOTHING about marriage.
You are correct, and that makes it a 10th amendment issue, completely up the the States and The People.
Which is exactly how it violates the Constitution! The powers of the federal government, which includes the judiciary, are few and limited. The federal judiciary has no place to interfere in the states' ruling on these matters.
I thought 10A was fairly clear. Guess not.
The traitorous judge should be impeached and convicted... which Congress and the Senate will do as soon as pigs fly.