You've got it all mixed up, Gelato. The federal gubmint is supposed to be limited to only those powers delegated to it by the states and the people. The question you should be asking is what constitutional right does the fed guv have to butt in? Show me that one.
First, the federal government currently exercises jurisdiction on marriage, by prohibiting a number of states from practicing polygamy as a requirement for statehood. The Supreme Court upheld the federal role over a hundred years ago.
Secondly, the people have the power to amend the Constitution as necessary. The kind of marriage amendment supported by defenders of traditional marriage clears up any ambiguity on this subject, and defines marriage as between one man and one woman.
Fred's solution here not only turns that idea on its head, but it creates constitutional protection for gay marriage and polygamy among the states. That is not conservative.