On Villian's Koi Fish Mother, I noticed under genre, shoujo ai was selected. In the entire novel, while there was some ambiguity with a male lead, the protagonists has no actual romantic entanglement with anyone. There is definitely no ambiguity with any female characters, and no romance in the novel. So, I went to deselect the shoujo ai genre. This is where the problem came up. The genre can't be deselected. I don't know if it was intentional by the site, so I figured that I would report it as a bug, just in case.
The genres of this novel are Comedy, Drama and Slice of Life. Shoujo Ai is not one of the genres within this novel. If you try editing the Genres, you'll see that Shoujo, Shoujo Ai, Yaoi, Yuri, Shounen Ai and Josei are all locked. As in, you can't add them to this novel. This was manually done by the mods to stop people from putting the wrong genres on the novel.
This isn't a bug. You can't expect the staffs to read every novels on NU. If any wrong details were on NU that you can't edit in the future, just submit a report through your NU user profile (there's a report button) and explain the story details so the information can be corrected.