Well no, it was not the whole reason, nor is it an invalid reason under the Constitution pact as established, and it was hardly the only grievance with the Northern states and Federal government. Anyone who's going to speak on this authoritatively should at least read the secession articles:
The Declaration of Causes of Seceding States Aside from the initial seven states, six more followed the Fort Sumter stand-off and Lincoln openly initiating war to force secession states, understanding that states did not relinquish state sovereignty and rights of free people upon becoming members of the federation.
Ordinances of Secession
The election of Lincoln signaled two primary concerns to the South; a president of a party with an anti-slavery plank who voiced the unsustainable condition of a half-free, half-slave society, and who was beholden to Northern interests at the continued cost and grievance to the South. The exclusion from new territories was central among Southern concerns, as was longstanding inequality regarding taxation and disbursement of it, Northern protectionism, and their economic future should abolitionist factions continue inroads into abolishing it.
As devil's advocate, even if the *only* reason the South seceded was slavery, it would still not be the reason Lincoln invaded the South. To the contrary, he spoke of the fact there was no constitutional precedent for doing so (abolition), that blacks were an inferior race and should be recolonized outside the country, and that the preservation of the Union was the only goal. Northern men didn't volunteer to fight for abolition.
It's impossible to have a historically factual conversation which omits this fundamental basis.