Last November Governor Jay Nixon of Missouri issued Executive Order 13-14 which required the Missouri Department of Revenue to accept joint tax filings from any couple who filed federal taxes jointly, including same-sex couples who were legally married in another state. That apparently got Rep. Nick Marshall's panties in a bunch because he immediately threatened impeachment. Now it seems he's made good on his threat and introduced letters of impeachment.
There's a very obvious problem with Marshall's way of thinking. It's part of the original U.S. Constitution, not even one of the amendments.
Full faith and credit shall be given in each state to the public acts, records, and judicial proceedings of every other state. And the Congress may by general laws prescribe the manner in which such acts, records, and proceedings shall be proved, and the effect thereof.
You see, it doesn't matter if they managed to get an amendment passed in the Missouri State Constitution that says, "That to be valid and recognized in this state, a marriage shall exist only between a man and a woman." The United States Constitution says that you have to honor contracts from other states, and marriage is a contract.
But, just in case the full faith and credit clause isn't enough to sway you, how about the 14th Amendment? You see the federal government is giving full recognition to same-sex marriages now. Seventeen states also recognize same-sex marriage. So, if you can go to some places and have full marriage rights, and then go to another place and not have full marriage rights, then you're not getting that equal protection of the laws.
So it turns out the issuance of Executive Order 13-14 was the only proper thing Gov. Nixon could do under the law.