Answer by TR Livesey:
Slaves were legally considered property, so the owner was free to do with their slaves as they pleased. There was a definite market for sex slaves, quadroon and octoroons that were passable.
As far as someone else raping a slave owned by someone else, ordinary property laws would apply: the other person could be held liable for any damages that the owner could prove in court.
According to diarist Mary Chesnut, relations between male owners and his slaves was not uncommon, that you could see their masters faces in all the mulatto children. She complained bitterly that Northern women has no idea of how terrible slavery was, since they did not live in humiliated silence like Southern women did.