Netorare Manga no Kuzu Otoko ni Tensei Shita Hazu ga Heroine ga Yottekuru Ken | Reincarnated as a NTR antagonist. Chapters: 1-10

Man who hates ntr, accidentally buys a ntr doujinshi, but reads most of it, like us who also hate ntr but still read it because we either hate ourselves or are secretly cucks, im the first option. Anyways, then he wakes up in the world of the ntr manga as the cucker but swears to be better and to not ntr the heroine