The philosophical principle underlying secession seems to be the right to control your property, and thus the right to free association; to refuse to enter an agreement.
So, the opposite principle, for rejecting secession, would have to be that you cannot refuse to enter an agreement; you are a slave to anyone who wishes to "contract" with you.
Taken to its logical conclusion, the case against secession can be reduced to absurdity.
For example, if you cannot refuse any agreement, you cannot:
-shop at one grocery store and then patronize a different store
-an abused wife cannot leave her husband; in fact, all divorce would be illegal
-you cannot move out of your neighborhood
-you cannot refuse to sell your house to a willing buyer
...and so on.
Does anyone else have any reductios to add? Have fun!