Wow. The way you phrase the question, it almost sounds like secession was the issue, rather than slavery.
Interesting theory you have there!
Why was any slavery acceptable?
You'll have to ask Lincoln. I wasn't making decisions for the Union Army back in 1861.
Well, from the south's view it WAS all about slavery, as they made clear over and over and over. But from the United States' view, it was about preserving the union, as they made clear over and over. You might start with Lincoln's First Inaugural.