You might want to reexamine that argument.
A parallel one would be to say, "If force is legitimate to prevent a bank robbery, then it is equally legitimate in perpetrating the robbery."
Force is legitimate, when other options fail, in defense of rights and in the service of good. It is never legitimate in the furtherance of evil or the violation of rights.
I'd agree if we were discussing a bank robbery, kidnapping, or slavery today. But secession was not defined as either thing until after the fact - when the winning side defined it as such.
Today, with just short of half the states in the union currently suing the feds or being sued by them precisely over 21 century interpretations of the Constitution, and with secession creeping back into the debate - it is again a subject for consideration.
How many here who insist that Lee and all his forces were traitors are being called just that today because they oppose Obamacare, bowing to foreign potentates, government by fiat, and a metastasizing deficit?
And...
I wonder how many southerners thought that "the army would never turn its guns on the people" - or on the people's militia??