When No Means Yes

I was recently browsing through manga on a scanlation site when I came across a maturity warning for Haou Airen. It read "WARNING: This series contains strong "consensual rape." Consensual rape? What does that even mean? It's like jumbo shrimp, an oxymoron. Luckily, the author of the warning wasn't completely oblivious to the ridiculousness of the statement. But why use the phrase in the first place?