What about those states who did not ratify the Constitution but were instead admitted by a vote in Congress? In other words all but the original 13? It was hardly an agreement on their part; in fact the Constitution doesn't give the people in the territories any say in the issue of statehood or not. How can they void an agreement that they didn't agree to? If your point is that for the original 13 states leaving should be the same as joining, an agreement entered into can also be opted out of, then for all the states that came later shouldn't leaving require the agreement of those states that let them join in the first place?