The founding fathers as slave holders argument is a variant of Reductio ad Hitlerum.
It's the absurd idea that anything Hitler did is automatically bad, for example the Nazis created Volkswagen, so therefore VW is evil.
In the case of the founding fathers, the fact that they were slave owners is supposed to mean that everything they did was bad and should be changed. Often it's used to justify the illegal behavior of current politicians because the old rules they are breaking were written by slave holders. The next time you encounter this argument, apply the same standard to Bill Clinton and watch them take an ideological 180 degree turn. Sometimes they switch from slave ownership to using the behavior of George Bush (senior or junior, take your pick) as justification. Inform them at that point that you are not a Republican and then the real olympic backpedaling begins.