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.