As someone who adores the game, I completely agree that it should be marked as NSFW. Barring any thought towards the ever-present themes between it's two main characters, it also simply has significant gore. I uploaded/suggested the game and its sheets so far, i'm just, ahh... not sure... how to mark it as NSFW. if it's something i myself can do...

This game has some pretty nasty themes and undertones, including but not exclusively, Parricide and Incest. I feel like an argument could be made to label this game as NSFW because of the game's subject matter. The games not necessarily porn, but i was kind of shocked to see it here.

From what I can tell, the game's creator has basically welcomed the perspective of their game being "The Incest Game", doing stuff like making fun of people who find the subject matter disgusting or appalling on X Dot Com.

The game is also known for trying to push a lot of social boundaries for the sake of shock value, if more imagery from this game is to be released I can imagine the NSFW tag being applied.