I always believed that he did father some children by slaves. What would make him any different than any other master back then? He was a human being after all. Doesn't take away from what he did for this country though.
I have no trouble believing that a lonely widower could find some solace, perhaps even love, with a woman who was half-sister to his beloved late wife, and was said to look very much like her. We tend to focus on the fact that Sally Hemings was "black," and forget that she was apparently quite pale and had Caucasian features, as she was the daughter of Jefferson's father-in-law.
Our founding fathers don't all have to be perfect paragons to be worthy of our highest esteem and gratitude. If a man loves his late wife's half-sister, it may not be exemplary behavior, but is it the crime of the century? Is it so wrong that we have to expend millions of words protesting against the accusation?