While I didn't see or here it myself, my understanding is that John McCain opposes the Federal marriage amendment because it isn't necessary, and it infringes on the rights of the States.
Tell me again why this guy is qualified to be President? To be sure, McCain's got the sophistry down necessary to be a successor to Bill Clinton. Then again, we knew that when he supported "campaign finance reform" allegedly justified by the corrupting influence of money in politics, but refused to identify a single colleague allegedly corrupted by money in politics.
As for the marriage amendment not being necessary, I know he was out of the country for awhile, but surely Roe v. Wade must ring a bell. You know: the decision where the Supremes struck down the laws of fifty states regulating abortion? So either McCain is oblivious to the judicial threat to marriage, or he's simply ignoring it. Either way, his notion is disingenuous.
As for "the rights of the States," they're entitled to their own laws, not their own language. In English, marriage is the union of one man and one woman. That certain state and/or federal judges don't understand that fact is one reason why it's both necessary and appropriate.