Counterpoint: many of the founding fathers were crazy sexist racists, yet we still hold their other opinions in quite high regard.
The immorality of slavery was recognized hundreds of years before the founding of the US. It was abolished in France in the 1300s, in Sweden in 1335, etc. It was never really legal in England and that fact was recognized in 1706, about 40 years before Thomas Jefferson was born. The founders' condoning slavery was a deeply immoral political expedient that was recognized by many as immoral even contemporaneously.
But that's besides the point. The fact that the founders were individually contemptible people doesn't mean their ideas were wrong. You can agree with many of the things Thomas Jefferson said without personally condoning slavery and rape.