Well, for obvious reasons, at the time
I'm putting this here because it seems the least wrong location. If it should be somewhere else, just move it. I don't mind. On to the actual question. In the antebellum South, slave owners had sex with their slaves, with or without the cooperation of the slaves in question. That's not in doubt. But I haven't been able to find any sort of information on how common a thing this was. Actually, I can find quite a few articles, but nothing resembling a consensus. Were such relations common, uncommon, open, hidden? I'm assuming male owner and female slave for purpose of this question.
Well, for obvious reasons, at the time the realities were obscured by law, custom, and tradition.
Because the enslaved were
legally property, they did not have any rights to self-defense, and
all black women were sexually available to (essentially)
any white male. African-American marriage and parenthood was not legally recognized, other than establishing the status of enslaved for children, and there was no virtually no recourse for sexual abuse in the courts; when it (occasionally) became a factor in a legal case, as in the
Celia case in Missouri, a defense on such a basis was - generally - disallowed, or failed to secure a not guilty verdict.
Children of mixed race, however, were found everywhere in the south - the 1860 US census listed almost 600,000 people as
mulattos, which compares to the total of 3.5 million enslaved in what became the rebel states. A tiny number of whites recognized their children and attempted to provide for them; some encouraged escape and passing, but the vast majority of enslaved children of mixed ancestry were simply worked and sold like any other slave. The "fancy trade" in "high yaller" women was generally understood as a reality, as well. Selling one's daughters into prostitution was profitable, if the father could stomach it.
Puddn'head Wilson was published in 1893-94, but even as a tragicomic take on the realities of slavery, it is sharp enough; Clemens knew of what he wrote.
https://www.gutenberg.org/files/102/102-h/102-h.htm
The larger issue of sexual abuse of enslaved women (and men, presumably) was understood in antebellum America, north and south; it is what the Brooks-Sumner affair was really about.
Best,