I will agree with you that Slavery was legal at the time. If the South had not seceded, it would have been continued, at least in the states that had it. One of the main items that led to the Secession was the Southern fear that they would not be able to expand slavery into the Territories.
However, there was a large and growing percentage of the population that considered it immoral. Please note that all of the Northern European countries, most importantly Great Britain, had outlawed slavery and the slave trade decades earlier. The fact that the Confederate States allowed slavery was the main reason that Great Britain never recognized it. I believe that the fact that slavery is almost universally condemned is the reason why most Southern Apologists find it so hard to admit that this was the main, if not the only, cause of the Secession and cloak it in States Rights, or tariffs, or self-determination or, most hilariously of all, claim that they reason they seceded was unimportant.
As to the issue of whether the South had the right to secede at all, I will only say that there were two sides to this discussion, and a vigorous debate was had by both. The debate opened on April 12, 1861 in Charleston Harbor, was forcefully rebutted in July 1863 in Gettysburg, Pennsylvania and Vicksburg, Mississippi, and was finalized at Appomattox Courthouse and Bennett Place in April 1865. In the last 150 years there has been no legal challenge to results of this debate, even after all of the Southern States reclaimed their positions in the United States House of Representatives, Senate and the Judiciary.
I will close by going back to my main point, which is, if the people (at least those who mentioned anything at all )who seceded at the time had no problem claiming it was due to slavery (see Articles of Secession for Mississippi, South Carolina, Georgia, Texas, and Florida), why do some folks nowadays have a problem with it?
“The fact that the Confederate States allowed slavery was the main reason that Great Britain never recognized it.”
Hogwash. Great Britain recognized the United States, which allowed slavery up until 1865. Or, are you saying Great Britain did not recognize the United States until 1865, when slavery was finally abolished by the 13th Amendment?
And I assert that it would have continued to grow, that it was in fact, unstoppable. That the war was unnecessary, and that time would have done the job anyway.
Tracking the progress of the abolition movement it becomes apparent that it's an exponential growth. It would have slowed in the South because they were so heavily invested and dependent upon it, but the Social attitudes would have continued to erode it until it was gone.
I believe that the fact that slavery is almost universally condemned...
In Christian nations.
most hilariously of all, claim that they reason they seceded was unimportant.
"Irrelevant." Get it right.
I will only say that there were two sides to this discussion, and a vigorous debate was had by both
Force is not a debate. Ultima ratio regum, is not an actual argument. It is merely a reiteration of the principle that "Might makes right"; That the Strong justifiably conquers the Weak. It is, in fact, a Justification for the principle of Slavery. It is the very foundation of it.
In prosecuting the war, the Union affirmed it.
why do some folks nowadays have a problem with it?
Because it loses the larger point in all the emotional noise. Of course, this is the intended function of steering the discussion into that cul-de-sac.
An honest, and logical debate does not suit their ends.