Kannagi caused much outrage and extreme reactions when the manga hinted back in November that the titular goddess may have consummated a previous romantic relationship long before she appeared before the male protagonist in the first chapter, as long-time readers of this site may recall.
An erotic doujinshi by Hozumi Takashi of circle P-FOREST (who seem to have a very limited online presence), published at Comic Market 75, confronts the main couple with the problems this implies for Jin.
A translation is now available for your viewing pleasure on the Channel.
The doujinshi in question is of excellent quality, and all three main characters make their appearance with even Zange-chan included in a few pages — indeed, she plays a central role in advancing the plot leading up to the erotic scenes, as she decides to challenge Nagi-sama to take their popularity contest to the next level.
His Zange-chan doujinshi, published simultaneously with this Nagi-sama volume, continues the storyline but ends on a cliffhanger — please also look forward to the forthcoming Channel translation of the next chapter in what may turn out to become a wonderful series.