Such a referendum is an example of a democratic process. It is not unconstitutional.
Now, if 70% of the people oppose homosexual marriage, I suppose one could say that this would ignore or defy the interests of many interested parties. But that's OK. Again, it is not unconstitutional.
A state-wide vote on secession would be a state-wide referendum. There is nothing inherently unconstitutional about that process.
There's nothing legally binding about it with respect to its neighboring states either.
No man, no association of men, no state or set of states has a right to withdraw itself from this Union, of its own accord. The same power which knit us together, can only unknit. The same formality, which forged the links of the Union, is necessary to dissolve it. The majority of States which form the Union must consent to the withdrawal of any one branch of it. Until that consent has been obtained, any attempt to dissolve the Union, or obstruct the efficacy of its constitutional laws, is Treason--Treason to all intents and purposes.From The Federal Pillars series published in Massachusetts Centinel it could be argued that the individual States have a vested interest in preservation of their Union amongst themselves. Suppose only 9 states would have ratified? What fate would befall those states not ratifying? Would they have been forced at gunpoint into the Union?
After the Revolutionary War, economic difficulties in Massachusetts resulted in many farms that were heavily in debt being seized by their creditors and often sold for a fraction of their value. The farmers and working men of Massachusetts who were unable to pay their debts were sent to debtor prisons and would not be released until their debts were paid. The state legislature of Massachusetts responded to this economic crisis in a very inadequate manner such as increasing court costs and raising taxes.
As a result, mobs of farmers and workers took matters into their own hands in August 1786 and barred access to the courts of several towns such as Pittsfield and Northampton. Among the important leaders of this rebellion was Daniel Shays, a Revolutionary War veteran from Pelham, and the rebellion came to be known as Shays Rebellion. The rebellion pointed out the weakness of the Articles of Confederation for governing the United States. In order to prevent such anarchy in the future and to strengthen the central government, the Philadelphia Convention convened to draft the Constitution in the spring of 1787, just a short time after the end of Shays Rebellion.
After seceding, the rebel-States demanded the surrender of federal forts within their geographical boundaries; President Buchanan refused; southern state troops subsequently seized them through force. At Fort Sumter, South Carolina troops repulsed a supply ship trying to reach federal forces based there. The ship was forced to return to New York, its supplies undelivered. President Lincoln alerted South Carolina - in an attempt to avoid hostilities - that he intended to deliver supplies to Fort Sumpter. South Carolina, however, feared a trick; the commander of the fort was asked to surrender immediately. Ft. Sumpter's commnander, Anderson, said he'd surrender after supplies were exhausted. That stipulation was rejected and on April 12 shots were fired on the fort. I believe that such action falls squarely under the purvue of Arcticles I, Sec. 8.15 & IV Sec. 3.2, 4.2 of the Constitution.
After several additional declarations of secession, an outraged Northern public clamored for a march against the newly declared Confederate capital at Richmond, VA, with the naive view that an early end to the conflict could be had. General-in-Chief Winfield Scott, yielding to the public's clamor for action, sent inadequately trained and untried troops into harms way. The results of that debacle left no doubt that a very long slugfest was to ensue before the matter would be settled decisively.
The fact of the matter is that the States individual self-interests lie with the strength or weakness of their Union collectively as a whole. If the States collectively decide that its in their self-interest to discharge a member from their Union, so be it. However, unless the several sovereign and united States recognize the secession, then the seccession is merely insurrection or rebellion and must be put down (with extreme predjudice if need be).