We have very little reason to believe that incest was particularly prevalent in slave communities, either spontaneously or by imposition from the white planter class. The idea that incest is a uniquely and profoundly African-American issue is an old one that originally sought to explain urban black poverty and anti-social behaviour as the result of degenerate family relations; it has been taken up by modern racists to try and make the case for genetic deficiency in the African-American population. Neither kind of argument is rooted in historical (or scientific) truth.

Certainly, incestuous relationships did occur - but in the case of slavery, these relationships could have been between white planters and slaves, not just between slaves themselves. If a planter fathered a daughter by a slave, then either he or one of his sons might seek sexual relations with that daughter later in life; incest was not a phenomenon exclusive to forced breeding practices. And whilst as you say some masters may have promoted forced incestuous relationships, as you have identified, this would have been far from the norm. Incest is fundamentally a social construct, and in contemporary white society it was very much a serious taboo just as it is today; whilst some planters may have been happy to forcefully impose incestuous relations on their slaves, it is reasonable to assume that they would by far be the exception, not the rule.

Furthermore, for the argument you encountered to be correct, it would require either forced breeding to be extremely common place, or for incest to occur frequently and spontaneously in slave communities. Neither was the case.

Whilst the historical record is rich with examples of masters who intervened directly in the sexual lives of their slaves, the vast majority of these relationships do not appear to have been incestuous. With the case of slave breeding, the literal forcing of two slaves to reproduce is one extreme along a spectrum of measures planters took to promote slave reproduction; such forceful imposition would have had to take place on an impossibly large scale.

In black communities, slave narratives referring to incest indicate that the practice was very much a taboo among slaves; incestuous relations imposed on them were considered particularly appalling. Joshua Rothman's book Notorious in the Neighbourhood contains one memorable story in which a slave woman murdered her master - also her father - for trying to have a sexual relationship with her; in her mind, the consequences of murdering her white owner were preferable to enduring a sexual relationship with her own father.

So whilst there were certainly incidents of incestuous relations, they were by no means the norm - either in slave breeding practices or in slave sexual relations. Where they did occur, they could have also occurred across the colour bar, not just in the context of voluntary or imposed intra-slave relations.