In theory, there's nothing controversial about a federal judge striking down an amendment to a state constitution on the grounds that it violates the federal constitution; the U.S. Constitution says that federal law is "the supreme Law of the Land... any Thing in the Constitution or laws of any State to the Contrary notwithstanding." (Article VI, cl. 2).
What is, of course, controversial (to say the least) is the idea that banning gay marriage violates the U.S. Constitution; but that's a separate question than the one you asked.
Where does the 10th Amendment fit, in this, say that all that is not granted to the federal government is reserved to the states?? And what about the fact that the States created the Federal government?
I’d say judges have overreached their authority.
Per the 10th, the states SHOULD be supreme in all that is not EXPLICITLY granted to the Federal Gov. in the U.S. Constitution!!