How can 3% of a population be the origins of THE ISSUE in any matter? It was much more than that.
A sense of nationalism? (or stateism in this case) Economic freedom from an industrial counterpart? A flat out disagreance with how things were being run?
None of these are "legit" to you, yet that's the cause many in the CSA troops wrote home about.
Troop write home about alot of things.
During WW2 troops were not writing home talking about their fight for freedom, or against Nazism.
The political decisions that led to the Civil War were about slavery.
Your appeal to the numbers of people who held slavery is irrevelant.
The fact is, the slave owners made their 'peculiar'institution a Southern issue, and men went to fight to protect the 'Southern way of life' which included slavery.