This question is proving challenging, in terms of finding the citation requested. Here's what we've got so far.
Angela Davis mentioned this (what we should probably call) allegation in a 1975 Ms. Magazine article. If this is not the column in question, perhaps your writer heard about it here.
In an 1899 book by John Spencer Bassett, Slavery in the State of North Carolina, on page 28 he relates this anecdote, with a very relevant footnote:
In regard to the slave's legal status a curious case has come under my notice. The late Dr. John Manning, widely known as Professor of Law at the State University, told me that Judge Ruffin, the senior, told him that a case was once decided in the North Carolina Supreme Court in which it was held that a white man could not be convicted of fornication and adultery with a slave woman, because such a woman had no standing in the courts. The case, said Judge Ruffin, was decided early in this century, but it was agreed that in the interest of public morality it should not be published.1
1 Inquiry of the Clerk of the Supreme Court fails to discover the papers in reference to the case; but since there is no other index to the Supreme Court cases than the printed reports it is quite possible that the papers are preserved, but so lost among a vast number of documents that only a long and careful search would bring them to light.
Our own inquiries over a century later are achieving similar results. The reference department of the University of North Carolina Law School told us "I can tell that finding this supposedly unpublished case (if in fact it exists) wouldn't be easy. We have all of North Carolina's published Court of Appeals and Supreme Court cases, but it's only been recently that unpublished cases have been made easily available. We're a public library, so [anyone] can come here and use all the resources we have to find the case he or she is looking for, but ... [it would] be a very challenging and difficult task."
An inquiry was also made to the North Carolina Supreme Court Library, but due to a current move most of their material is in storage until mid-April 2007. If further information comes from that source it will be posted here.