In general, I would say the founding fathers weren't unaware of the moral compromise of their position. They were hanging out with Benjamin Franklin and John Laurens, so it's not like they hadn't heard the arguments. It's undeniable that they deliberately tried to tamp down abolitionist sentiment because they felt the revolution wouldn't succeed without the Southern states and they wouldn't agree to abolishing slavery. It's also clear that this strategy of "just don't talk about it" became ongoing American policy that led to the Civil War. At the same time, they could've explicitly enshrined slavery, as the Confederacy did, and they chose not to do so, but to preserve the dissonance between America's aims and it's condition.
It's not a particularly bright moral line, but I do think the difference is clear between a man who participated in the slow boil of American slavery versus a man who explicitly fought to change the country to make permanent and unchangeable the peculiar institution. Washington was pretty bad, in terms of slavery, but he left the door open for a country where it could be abolished. Lee wanted to hammer that door shut for all time.