"Whenever any Form of Government becomes destructive of those Ends (Life, Liberty and the Pursuit of Happiness), it is the Right of the People to alter or abolish it, and to institute new Government, laying its Foundation on such Principles, and organizing its Powers in such Form, as to them shall seem most likely to effect their Safety and Happiness..." Declaration of Independence, 1776
This statement from the Declaration of Independence would seem to run contrary to CigarPrimate's "Perfect Unon" theory. Notice the Constitution just said "in order to form a more perfect union". It doesn't imply a perfection so good that nobody would ever want to leave. More perfect than what?
I submit that the answer to THL's original question is still undetermined and is mired in the debate of lose versus strict construction of the US Constitution. The US Constitution does not specifically say that you can't leave the club once you join. It also does not say you can ever leave. Hence the difference of opinion.
Some states thought they could leave because they never agreed not to so they held legal constitutional conventions according to their state constitutions and voted to secede. If they were right, they were then independent nations that were invaded, conquered, and annexed. If they were wrong, they were a rebellion and were squashed.
After the Civil War, the states that seceded were required to draft new constitutions that formally repealed their previous secession documents and had to ratify the 13th amendment abolishing slavery. If their secession was merely a rebellion, why go to this formal extreme of repealing the secession document and re-admitting a state that never left?
In the case of Florida, a new state constitution was drafted in 1865 but rejected by the US Congress and the state was placed under military rule until 1868 when another state constitution was accepted.
The following was added to get the military occupation to end: "Section 21. This State shall ever remain a member of the American Union, the people thereof a part of the American nation, and any attempt from whatever source, or upon whatever pretense, to dissolve said Union, or to sever said nation, shall be resisted with the whole power of the State."
In 1885, Florida again created and ratified a new state constitution. The above section about never leaving the union was removed and it is still gone today in newer versions of the Florida state constitution.