Yes, government already regulates marriage. I never said it didn't. The horses are out of the barn and it's too late to close the doors. If the federal government wants to in the future, it can continue to regulate it and declare that all states must accept same-sex marriage; all with only a majority to pass any other legislation. All that's needed now to force all states to accept same-sex marriage is a decision by the federal government (a majority decision by the Supreme Court or legislation from Congress) to further regulate it. The FMA protects states from imposing a radical new regime of marriage on them by the federal government. To change this afterward would require a new amendment instead of mere legislation. Basically all the FMA does is say what 'marriage' is. If you use a different label, you can give same-sex marriage all the benefits of anyone who is considered married, but only in that state. If another state *chooses* (is not force to) it can recognize that 'marriage'.
What would happen if this was passed is that the communists running the homosexual agenda movement would pass another amendment changing the wording of the marriage amendment...and that will give even more power to government. It's not to hard to imagine that this will all lead to government having the power to tell you whom you can't marry and who you must marry.
In the end, government will do what it can get away with. An amendment merely makes it more difficult to get away with it. You are concerned about things which are already a possibility, but are unwilling to do anything to mitigate these possibilities for fear they will make them a possible. If you think this sounds a little bizarre, that's the point.