If true it doesn't exactly speak all that well of them, does it?
They were people of their time and place, and what they were taught to believe did not seem wrong to them. I would like to believe that I would not have gone along with the crowd, but I cannot honestly say I would have had I been raised to believe that what they did was acceptable.
Never the less, they joined the Union with the premise that their peculiar institution would continue, and this position represents their free will choice on the matter. What the Federals did to force passage of the 13th and 14th amendment does not.