Modern mores.
There is a tremendous tendency among critics and historians in general to interpret behavior in light of current standards of society, law, mores, folkways and custom. That Sally Hemmings was a slave and 15 years old and therefore off limits to the 50-something Jefferson is entirely revisionist standards. As a slaveowner Jefferson was within his legal rights to do literally anything including kill her. Economic considerations, humanistic considerations and manners precluded anything so drastic, but runaway and recalcitrant slaves were sometimes abused or killed. Several of the founding fathers owned slaves, many were inherited, and many were emancipated from the estates; very humane for the period.
It is frequently overlooked that the whites in the early 19th Century started the abolition movement and persued that philosophy to its conclusion --- Civil War; and triumphed.