The Union of the Crowns (United Kingdom) was not "destroyed" when 13 colonies left it, and the Union of the Colonies would not have been destroyed either. To claim it would have been is just hyperventilating.
Secondly, the right to leave the Union (as articulated in the Declaration of Independence) is not conditional. It doesn't matter what someone's reasons for leaving are, they have the right to do so regardless of their reasons.
You are not the moral police, you don't have some God given moral authority to decide that rights are only valid if they meet with your moral approval.
The only thing required for them to exercise the right of independence was the belief that the existing government no longer served their interests. They so decided in orderly elections, and they chose to leave.
The rest is just screeching from the people who were losing the money that came from their export production.
The Union was like a balloon and it is no longer a Union if you can unilaterally leave it. That is a mere Confederation which the Constitution rose above. But even that document stated the Union was perpetual.
I can’t believe you believe the Civil War was of the same nature as the Revolution. Of all the weak, vacuous arguments that is the most weak and vacuous. There was no union in GB it was a KINGDOM; no colony had joined the Kingdom voluntarily. None had any representation in Parliament. Not only did the Southern states have representation they had controlled the government until Lincoln and the Republicans came on the scene.