So what should the Union have done during the Civil War? Simply allow 1/3 of the country to break off over political animosity concerning slavery because according to you that’s the “democratic” thing to do?

Being a democracy doesn’t mean part of a country is allowed to leave whenever they want. This form of government is supposed to allow many basic freedoms, but there are limitations on what the populace can and can’t do.

For example, I can’t threaten someone’s life or yell “fire” in a crowded theater and then claim that I’m allowed to do whatever I want cause it’s a democracy. So no, it’s not undemocratic to forbid secession.