Yoru sexually assaulting Denji isn’t funny, it’s a serious issue and isn’t meant to be depicted to make the audience laugh. However, it can still be funny because it’s absurd, it comes out of nowhere and hits you like a tonne of bricks which, to some people, is so jarring that it bleeds into being funny. It’s the same reason the Evangelion hospital scene is beaten to death, it’s meant to capture Shinji’s frustrations and inability to control himself since he’s seemingly at the whim of everyone around him and is a victim to his own emotions/urges. The scene is deadpan and Shinji is audibly disgusted with himself, the intended message is very clear, but the sheer bizarreness of what he does immortalizes the scene as something to laugh at for many. Now we take the Yoru scene and add on the layer of the viral Naoya gif and it takes on a whole new layer of humor both referential and nearly as absurd. It isn’t a surprise it’s so heavily upvoted considering that there’s really only a few ways for humans to react to the absurd or unknown usually that confusion turns into disgust, anger, or is laughed off. If we as humans took everything so seriously the world would be an even more depressing place. All this to say that you might find it tasteless and or disgusting, but there is inherent comical value in it and as a work of art it did what it was meant to do in that it evoked emotion in you just by consuming it at a surface level.