Have you read any of the secession resolutions? They all make it quite clear that Slavery was the reason for secession.
Only three of them cite slavery. Most of them simply say we secede. There is no question slavery was used to gen up support for secession among the Southern population after the decision to secede had been made. So what? The constant fallacy in these debates is in conflating secession with the cause of the war in the first place. The war was entirely a Northern project and the North pursued the war for its own ends, slavery not being among them.