Does Mitt Romney want to annul existing same-sex marriages?

The official 2012 presidential platform of the Republican Party calls for a constitutional amendment to ban same-sex marriage and nullify all existing same-sex marriages in all of the United States. This amendment has already been defeated in Congress. Presumably, Romney would sign it into law if it passes. Remember, though, that when he was governor of Massachusetts, he signed that state's marriage equality bill into law. Similarly, he says he wants to get rid of "Obamacare", but he also signed a similar bill into law in Massachusetts. The bottom line is that Romney, if president, probably would not be able to eliminate same-sex marriage.