In my opinion there isn't a question of whether a state may leave or not. Nothing in the Constitution forbids it. The question is the manner in which they choose to leave. The Southern states seceded unilaterally. When they did so the walked away from financial and treaty obligations leaving the remaing states to shoulder the responsibility for all obligations in full. The took with them literally every piece of federal property they could get their hands on without compensation of any kind. For a brief period of time Mississippi closed the river to all traffic bound for the North, cutting off much of the United States from access to the sea and placing an economic stranglehold on them. So my question is, why do you feel that the Constitution protects only those states leaving and offers none to those remaining? Why can the seceding states use the Constitution as a club to beat the other states up with? Don't the remaining states have any rights at all?
Secession should be allowed, but only with the approval of both sides. Requiring this makes sense because only then are the interests of both sides protected and only then are all matters of potential disagreement ironed out before the partitioning. And I believe this is implied in the Constituiton as well. Approval is needed to join. Once in, approval is needed to partition a state into to or remove a state entirely by combining it with another state. In fact, Congressional approval is needed to for a state to change it's border by a fraction of an inch. Why should leaving be any different?
Because it's not in the Constitution. Period. If it's not in the Constitution, it is a right delegated to the several states by the Tenth Amendment. That's the end of the debate.
If you think it's a bad deal, amend the constitution to require approval of Congress before a state can leave the Union, but as it stands now, it IS, in fact, a unilateral power of the state.