As someone who does genealogy and collaborate with other genealogist from time to time, there are two things, despite being taboo and socially unacceptable, that can be found in virtually
all families (black, white, or otherwise) - Incest and Homosexuality. People don't typically want to talk about them but it is so prevalent that it is even a non-factor in genealogical circles.
You have a couple of scenarios on how this happens. As it relates to incest, people are naturally clannish and not too long ago, not very mobile. In the south, especially, black families typically lived around the same families for generations. These families were your dating pool. So it didn't take too long for the Jenkins, Johnsons and Williams families in Magnolia, Mississippi to become interconnected through marriage. And because, unless one enlisted in the army, you had few families moving out or coming into the area, naturally each successive generation linked up with what was only available to them - which was the local dating pool. If your people been in a specific area long enough, you would have been already related in some way to most of the black families in the area.
Then you had some families who did it as a survival mechanism. My family for instance were free people of color from the Midwest and Mid-Atlantic regions. They actively engaged in incest as they wanted to maintain their "free blood" status. Most of the free families in the area we lived in had been marrying each other since the mid 1600s. By the 1700s, they were all related. The thought was, "if we continue to marry other families documented as being "free" for a number of generations, this will create a buffer in the instance that a white man comes along and start claiming one of our people as being their slave." You see you couldn't just come around and claim such and such as a slave when his family, the Locklears of Lumberton, North Carolina, were known by all the white families in their area as being a free Negro family. When you get to about four or five generations up in my family, it becomes
.
All this to say, in the court of public opinion, incest is viewed unfavorably and will even be denied by most people. But I would bet my left hand that if I checked the family trees of every single poster in this thread, I'd find evidence of incest. It was just that common. People are more mobile nowadays so the external factors that contributed to active incest are not present but familial behaviors and characteristics are passed down from one generation to the next so I wouldn't be surprised if people currently still engage in it with the prevalence that it occurred in the past. Again, it's a non-factor with most historians and genealogist who are able to see these same behaviors occur across a multitude of families generation after generation. The average joe will not see human behavior from this vantage point so quite naturally find it "morally" opposing in some way. Human behavior is human behavior.