Show us where the constitution bans unilateral seceding.
Article 1, section 10
"1: No State shall enter into any Treaty, Alliance, or Confederation;"
1. No mention there of secession. And anyway, once a state left the Union, the constitution did not apply to them. Article 1 applies to states still in the USA that try to make deals with foreign countries.
2. It's preposterous to say the founding fathers would ban secession right after they fought a war to secede from england.
3. The constitution never mentions secession directly and so, by the tenth amendment, each state has that right.