Romney's Double Standard

The Massachusetts governor has described John McCain's federalist position on gay unions as "disingenuous." But what is Romney's position on abortion? Here it is:

The federal system left to us by the Constitution allows people of different states to make their own choices on matters of controversy, thus avoiding the bitter battles engendered by ''one size fits all" judicial pronouncements. A federalist approach would allow such disputes to be settled by the citizens and elected representatives of each state, and appropriately defer to democratic governance.

As Jon Rauch puts it, "So there's room for moral variance on whether to slaughter unborn children, but not on whether to marry gay couples." I think Mitt Romney needs to clear this up, don't you? Why is he not in favor a federal constitutional amendment to ban all abortions?