Why were all black families denied from the temple before 1978? Did the church not want black families to be sealed to each other?

While we don't know the answers to all things, we can find solace in knowing that this Earth life is a simple blip on the eternities. God has promised that all of His blessings will be made available to all of His children eventually, now or in the eternities. Sometimes we see that happen here as with the revelation that all worthy male members can receive the priesthood. Other times we have to do our part, working in faith knowing that God will fulfill his promise. For the Church's official explanation please read this- https://www.churchofjesuschrist.org/study/manual/gospel-topics-essays/race-and-the-priesthood?lang=eng

Nice answer Mark, it reminds me of why we don't know why Jesus did not send the gospel to the gentiles during His ministry, but years later sent the Apostle Paul beyond the Jews. I am very proud of the church's anti-slavery stance as early as July of 1832 IN a slave state (Missouri) and founder Joseph Smith's anti-slavery platform in 1844. We just try to love everyone as current prophet, Russell Nelson is demonstrating with his inner-city educational training programs in cooperation with the NAACP that he spoke about at their annual conference. We can follow his example and be more Chrst-like.