I don't see how it could. Federalism can work whether the states and federal government recognize and define marriage or other relationships or not. If the central government defines same-sex marriage and obligates the states to accomodate it, it is no different than a bunch of other federal mandates to the states. If the separate states choose different definitions for the word, somebody does have to resolve the legal consequences. I presume the rules would require equal treatment of equal relationships so that, for example, if only one state in our hypothetical federal nation defined marriage to allow a person to "marry" a parent or child solely to transfer health-care benefits or inheritance without some tax penalty, the other states would not be required to accomodate or recognize that relationship. They would have to agree in legislation which relationships would be "equal" under the law. I don't see any scenario where the principles of federalism are affected, just differences in how they are implemented.

Respectfully, for the state and Federal government, the principle of federalism is at the heart of the gay marriage debate. If fact, it is considered as important, if not more so (in light of the principle of separation of church and state) than the religious/moral debate on the issue. Here's why: As 'federalism' is the concept of how power is shared between the states and the Fed, it has become a general principle that when there is a conflict between state law and Fed law, Washington D.C. wins. Federal courts operate on this idea and the Civil War was fought over it and helped to cement it. Allowing states to recognize and license gay marriage cuts to the core of the balance of power in our federalist system. If a state recognizes gay marriage, but the Fed does not, is the state thus superceded? If so, a significant 'weight' of power has been 'slid' over on the 'scale' of federalism to the Fed's side. The states and the Fed are in a constant tug-of-war when it comes to their own autonomous power, whether we realize it on a daily basis or not. Anytime the power to "decide" the status of something (like gay marriage) is gained by one side, it is lost to the other. Rarely is there a true compromise. There is never a stalemate. Federalism is a constant work in progress. Consider the example of equal employment. If a married gay couple lives in a state that recognizes their marriage but the fed does not, does the company they both work for have to offer them the same benefits (health, otherwise) as a straight married couple? Assume the company has branches in many states - some that recognize gay marriage/some that do not. Who decides in court? Let's say the state court decides for the gay couple. Appeals take the case to the fed district courts who decide against the gay couple. If the debate makes it to (aka - is accepted to be heard by) the fed Supreme Court, they will not be having a debate about what is religiously/morally right or wrong. They will be in a hotly-contested debate over one thing: Federalism.

The federal government should not be in the marriage business. Article 10 - "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."

Its just marriage, does anybody else's marriage affect it?