The thing about gay marriage is, with the way the Constitution is written now, a state that makes it legal essentially makes it legal in all states. Fred Thompson says there needs to be an amenedment to fix that:
Republican presidential candidate Fred Thompson backs an amendment to the U.S. Constitution that would forbid judges from forcing states to recognize same-sex marriages that are legal in other states. Many Christian conservatives in Thompson’s party, however, favor an amendment that would ban gay marriage in all 50 states.
“You’ve got to be awfully, awfully reticent to go in and do more than is absolutely necessary in terms of a constitutional amendment and federalize what doesn’t need to be federalized to solve the problem,” Thompson says. “They understand that, appreciate it. I think they think that mine is a good approach. I can’t say that they think it’s a better approach than one that I might have, but I think that they respect my position on federalism.”
I don’t know about this one. It seems to me that marriage is a religious ceremony and should be dictated by the church, while civil unions are a contract between two consenting adults. I guess states can dictate who can enter into contracts with one another, but it seems to be a limit on liberty to me.

