The white slaveholders and Black preachers taught the slaves about sin. They were taught about raping, stealing and being disobedient to their masters but never were they taught to not fornicate. Fornication was a valuable resource for the slaveholders for producing a field workforce therefore the Black preacher knew that fornication was a taboo subject teaching slaves to reframe from sex.

Why wasn't the slaveholders concern about slaves sexual sins and what did the Bible say about fornication?

1 Corinthians 6:18 (KJV) 18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.

Fornication was not an issue when slaves were producing offsprings for the fields and slave markets. Fornication became an issue once slaves became free in 1865 and the births were detrimental to white farmers security because of revolts; and only then did fornication become a thou shalt not have sex outside of marriage upon the slave's freedom.

There were no sexual sins that the white man kept sacred as long as it was beneficial to him. The slaves were taught to ignore sexual sins. Then the preaching came about to control the birth of the new negro by the Black preachers who once remained silent about fornication. In these modern times history repeats itself. The Black man and woman are told fornication is a sin just to control Black births but just as the slaveholders knew then fornication is not a sin between two consenting adults which at the time included himself who was not committing adultery with a slave woman.