I am a strong believer in federalism. I go so far as to want a return of the state legislatures election of the U.S. Senators for thier states. That said, marriage is too fundamental to mess with. Protect it within the Federal Constitution. It’s wrong to see that protection as a “violation” of Federalism. By amending the Constitution, you amend what federalism means under the Constitution. Homosexual marriage is a Sodom and Gommorrah level corruption of the society. We are rushing right past late Roman levels of sin to Old Testament, fire and brimstone, unleash the flood types of societies. It’s worth an Amendment to stop homosexual marriage.
If that's your stance, no offense, you're not a strong believer in federalism then. Your stance on federalism has limits when it comes to moral issues, something Mr. Madison (and the 10th Amendment) stated were issues for the separate and sovereign states.