But according to this book,
The only problem was . . . [the comparison] wasn't true. For the sake of a convenient argument against the monstrosity of abortion, we abandoned the clear teaching of the Bible on another subject--how slavery was to be understood.
Today, if a doctor who performed abortions "presented himself for admission to your church," of course you would turn him away. But if "this same church was moved back in time, and a man presented himself for membership along with three of his slaves," it would be a different story.
So there we have it. Abortion is less forgivable than slavery.
UPDATE: See Paul and commentary on "The Friendly Face of Slavery in Cary."