We should be careful to avoid presentism, applying modern moral standards to somebody who lived over 200 years ago. By the standards of his time, Franklin was unquestionably a great man.
Most of the "bad" people in history (e.g. Hitler, Pol Pot) acted worse than the standards of their time. Nobody's justifying them or their actions with this logic.
But I take issue with forgetting about this one aspect of Franklin's character because he was simply following the standards of his time.
The question is, standards for whom? Affluent white people? The standards certainly weren't acceptable to black Americans.
I know that I may sound nit-picky, but I think it's important to apply our current sense of morality to this period in American history. When we gloss over the uncomfortable details, we gloss over some of the lessons.