So much for democracy, surely if a referredum was held in a state and a signifigant majority (say 70%) voted to secede. Haven't the people spoken and made their wishes known?
So yes they should be allowed to leave if they have the support of the populance. Otherwise if you force them to stay against their will, why not change from the United States of America to the British States of America. Because in essence forcing them to stay would be little different than the British trying to force America to stay a British colony like they did 236 or so years ago.
When the South seceded, Lincoln's (as well as most of the Union's) primary motivation in forcing them to stay in was the fear that allowing them to leave would mean not only the preservation and expansion of slavery, but also the near-certainty of war at some future point. Remember that this was the age of empires, and that history up to that point had been one long progression of warfare. Suppressing the insurrection straightaway would be far less violent than fighting a full-on war later. Obviously, this took longer in practice than the North had hoped.
Today, if, as you say, seventy percent of Louisianans wanted to secede, we could be fairly confident that even if they did
, there'd never be a war between LA and the rest of the Union, for obvious reasons. Conversely, as Circus Peanut
notes, the practical and bureaucratic costs of establishing an independent nation would be prohibitive, to the point where it's virtually inconceivable that even a simple majority of any state would want to secede.
So, the proper answer to your question is really: there's no sense in making plans for crossing that fantasyland bridge unless and until we come to it.