One of the arguments you hear against secession is, "Even if the rest of the states agreed, there's no provision in the Constitution for secession."
Ignoring for a moment how adorable it is that you think what the Constitution says still matters, I contend that the mechanisms that would allow for secession have already been used, lots of times.
Firstly, Congress can cede sovereignty over territory, which it does every time a new embassy opens. The United States even ceded control of José Martí Park in Tampa, Florida, to Cuba in 1956.
Secondly, the federal government functioned during the Civil War without participation from several states. Congress still functioned without the states' delegations present, and a presidential election was held without counting electors from those states.
So there's no Constitutional provision for, say, Texas quitting the Union. Fine. But Congress can cede sovereignty of Texas to the Texas state government, and then Texas can stop sending members to Congress and stop participating in American presidential elections. How is this not the exact same thing as secession?
One could argue that Congress has no authority to cede sovereignty over any territory that has citizens, but isn't that what happened with Marshall Islands, Micronesia, and Palau? Why can't Texas become a state in free association with the U.S.?
The Honduran supreme court ruled out charter cities, but that wasn't quite the same issue. There, the Honduran government had agreed to remove territory from the jurisdiction of their constitution while still maintaining it as Honduran territory. I agree that this shouldn't be allowed (although, at this point, is it really going to surprise you to find out that America has already done exactly that to 2/3 of its people?). But if the Federales ever agreed to give up control (I know, I know [wipes tears of laughter from eyes], I've got to stop so you don't get kicked out of the library for laughing too loudly), there's no reason that anywhere in America can't secede.