I am constantly amazed and appalled that this can still be a debate.
Several Freepers have already posted the text or links to what amounts to signed confessions by the political leaders of the several Confederate states in regard to the centrality of slavery in their deliberations.
The fact that most Johnny Rebs were serving out of loyalty to their states and didn't own slaves is beside the point. Even mentioning the Constitution's lack of explicit text concerning secession by member states of the Union is also beside the point. Just because you might have the "right" to do something doesn't necessarily mean that you should do it.
At the end of the day, you still have the legislators of the Confederate states stating that they are acting out of concern for protecting the "peculiar institution." They don't cite census numbers about shifting demographics nor tarriff rates or other such things. In fact, the South Carolina legislators griped in their declaration of principles for their secession that the Federal government wasn't making the New England states enforce the Fugitive Slave Act. (I guess "nullification" is only a selective political right.)
I also remind you that South Carolina and other states seceded before Lincoln was even inaugurated! (Therfore, nothing he did as President caused these secessions-- only the fear of what he might do.) Hell, South Carolina began the process as soon as the election returns came in.
Finally, pointing out that Lincoln started the American Civil War believing that the preservation of the Union trumped the abolition of slavery in importance is only half the story. By 1862, it became obvious to Lincoln that Emancipation had to be a crucial war aim--the prerequisite for real victory in the long run. In fact, one might make an analogy to Iraq today. Deposing Saddam Hussein is only the first step--changing Iraq has to be done or we will be back at it again in several years (Nancy Pelosi, call your office.)