What makes you think the states wont see marriage the same way you do?
Most of them will. One or two liberal states should not be allowed to radically redefine marriage for the federal government when the other 40+ states are appalled at the idea. An amendment defining marriage as between one man and one woman would allow for a consensus throughout the whole United States, rather than let some crazy liberals force this nonsense on everyone. And we should say "no" to marriage by another name as well. We shouldn't be scammed by stupid word games.