Well, since they likely equated that with thier right to property, it would seem "valid" on it's face.
How could any one say that the south seceded for every other reason but slavery?
Some may. I dont.
Disclaimer: Because some people are so obtuse - my first statement above does NOT mean I agree with the reasoning - instead the argument itself would have been one that could be argued according to the laws and customs of the time.
Do you believe the south was right to secede over their right to own humans as property?