In life there are good reasons to do things and bad reasons to do things. People and governments do things for good reasons and they do them for bad reasons.
We all know this to be a self evident reality.
Just because you are allowed to make a decision doesn’t mean you ought to, if you are doing it for an idiotic or immoral reason.
And with any decision there are consequences. No one stopped them seceding. But there were consequences. I personally believe if they had done it for good moral reasons they would have ultimately wound up winning and would have been a separate nation.
Had they remained in the Union, the nation may very well still have slavery. Would that be a "good consequence"?
It takes 3/4ths of the States to amend the Constitution. If the 11 slave states held together, it would require a 44 state Union for 33 state to outvote those 11 states and ban slavery. This could not have happened until 1896 at the earliest.
Had the five Union slave states also joined with them, it would have required a 64 state Union to outvote them. We do not yet have 64 states in the Union.
The Union was a slave Union. It would have continued being a slave Union if the South hadn't tried to become independent of Washington DC's control.