there’s a website where you have to post the most messed up stuff and the mc is told to take a video by this girl and the girl starts having sex with another guy (B) and the girl starts beating B up while the mc is taking the video. And the mc got a boner instead of getting traumatised.
Dead tube?
http://www.mangago.zone/read-manga/dead_tub
I found myself quite enamored with the story! Yet, I could not help but feel that it might have been even more delightful had it stretched a bit further, perhaps weaving in a few additional side tales to enrich its charm.
Same.Honestly it smells like they draw it from a novel. I bet the novel has what you wish for.