Perhaps you should review your understanding of the Constitution. The Constitution limits the powers of the government. The pertinent part of Article VI:
The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.
Why was this necessary? Because so many of the original thirteen state were established as religious enclaves for persecuted Christians in Europe. Article VI forbid states from requiring office holders to be of the official state religion, for example, you didn't have to be Catholic in Maryland, Presbyterian in Massachusetts, or Episcopalian in Virginia to hold office. Remember that there were official state religions at the time the U.S. Constitution was drafted.
There are plenty of good reasons not to vote for Mitt Romney (NONE of which outweigh the fact that he is not Obama, btw), but the fact he is a Mormon isn’t one of them.
An individual can use any reason he or she wants. The Constitution has no limit on how the individual makes that choice.
I agree that there are lots of good reasons not to vote for Mitt and I don't intend to vote for him. While I am opposed to Mormonism, I wouldn't vote for Mitt if he sincerely came to Christ tomorrow. His Mormonism isn't the reason I'm not supporting Romney - it is his politics.
I don’t think we disagree on anything, aside from the voting for Romney part.