As to avoid link rot, it may be best to quote the first few paragraphs of the Morgan essay. It also serves as a useful starting point:

"Interracial sex took many forms in British America. In those cases where the male partner was white and the female black--the typical pattern for most of the eighteenth century--it ranged from deep commitment on the part of the white man to his black partner and mulatto children to the most outrageous forms of sexual abuse. Jefferson's behavior probably fell somewhere in the middle. It makes little sense to assert that Jefferson raped Sally or that their relationship was the functional equivalent of a loving marriage. A more nuanced picture, as evident in many of the relationships previously described, is possible. On the one hand, the relationship between Jefferson and Hemings, as with all slaveowner-slave relationships, was ultimately a forced embrace. Jefferson owned and controlled Sally Hemings. Sexual access to slave women was one of the prerogatives of ownership. The word used by Madison Hemings (as well as by Isaac Jefferson and James Callender) to describe his mother (and also his grandmother) was concubine, which in Samuel Johnson's eighteenth-century dictionary was defined as "a woman kept in fornication." The word means literally to lie together. When John Adams heard the allegation of Jefferson's liaison with a slave woman, he was not surprised, identifying it "as a natural and almost unavoidable consequence of that foul contagion in the human character-- Negro slavery." Modern notions of romance--seeing Hemings and Jefferson as America's premier biracial couple--should not be projected onto unions born of trauma, dependence, and constraint.

On the other hand, such a relationship did not have to be based solely on heartless domination; it might have involved a measure of affection. Even Thistlewood, whose sexual relations with slaves were often brutal and coercive, was ensnared in the complexities of a genuine, albeit severely asymmetrical, relationship. It is surely in the realm of possibility that a widowed, middle-aged white man (who had pledged to his wife that he would never remarry) could be attracted to a fair-skinned African American woman, who was almost certainly the half-sister of his late wife. Marriages to close relatives were common in early Virginia, and marrying or cohabiting with a former sister-in-law was probably not altogether rare."