You'll recall that slavery was considered during the ratification and drafting, and that it was patently obvious to many that the Constitution would never have been ratified if it forbade slavery. To characterize it as "clearly understood" as "legal" and "right" is ridiculous. It was necessary for half of the signers to look the other way on this one issue until it could be resolved in some other way... or else we would have become yet another version of balkanized Europe. The Great Compromise, repeated Congressional debates, numerous anti-slavery movements, rules for states joining the union that hinged on slave-owning laws, etc., all did their best to resolve the issue without tearing the nation apart, and they successfully stalled the inevitable for almost half of our history. You may choose to ignore this, but that does not mean it didn't happen.
I fail to see what is so "ridiculous" about what I had stated. Regardless of the reasons behind the recognition of slavery as a "legitimate" (for want of a better word) institution in the new United States, the fact is that slavery was permitted. What you've stated here is correct -- the founding fathers, in an attempt to secure ratification of the Constitution, simply delayed the inevitable.
But I would suggest this -- if the southern states in the 1780s and 1790s had known what would transpire in the 1860s, I'd guess that not a single one of them would have ratified the Constitution.
What made the whole question about slavery as it related to the Civil War patently ridiculous was that the Union had no moral authority to weigh in on the issue one way or another. The Union was no better than the Confederacy when it came to slavery -- the Union simply didn't need slaves to the same extent that southern states did.