Yes. And they knew it (they explicitly called it barbaric). But regardless of the revisionist history taught to the current generation; the founders did not ignore the issue in the Constitution. Nor did they embrace the heinous act of slavery. Knowing that such an issue would divide and destroy the new nation; they did the only thing they could do to insure its ultimate dissolution, by creating a system that would ban the slave trade after 1808. To quote the author of the Constitution (Federalist #42):
"It were doubtless to be wished, that the power of prohibiting the importation of slaves had not been postponed until the year 1808, or rather that it had been suffered to have immediate operation. But it is not difficult to account, either for this restriction on the general government, or for the manner in which the whole clause is expressed. It ought to be considered as a great point gained in favor of humanity, that a period of twenty years may terminate forever, within these States, a traffic which has so long and so loudly upbraided the barbarism of modern policy; that within that period, it will receive a considerable discouragement from the federal government, and may be totally abolished, by a concurrence of the few States which continue the unnatural traffic, in the prohibitory example which has been given by so great a majority of the Union. Happy would it be for the unfortunate Africans, if an equal prospect lay before them of being redeemed from the oppressions of their European brethren!
Slavery was self evidently wrong to some people, perfectly acceptable to others. It may very well be that abortion will become as reviled in the future as slavery is now.
The stain on us will be that we (as a society) condemned slavery while embracing abortion.