Article I, Section 10 and Article IV, Section 3 both make it clear that Congressional approval is required for changes in a state's status. Implicit in that is approval to leave as well. Or so the Supreme Court found.
Now, your turn to pull something out of your...whatever and tell us what allows unilateral secession.
'Clear', 'Implicit'. So in other words you nor the court could find any legislative power prohibiting secession, or anything laying out in detail any legal procedure regarding the same. I thought so.