Eroge Sekai ni Tensei shita Ore ga, Oshi e no Ai de Netorare Heroine wo Shiawase ni Suru.

"Suck it nicely, you weakling!" The one servicing that bastard's thing was my beloved girlfriend. There was nothing I could do before that scene... After starting to play an erotic game recommended by a friend, Mitsuki falls head over heels in love with the game's main heroine, Kotoha. But this game is a NTR game in which the heroine is seduced by the player's best friend, Ryou?! While searching for a happy ending with Kotoha, Mitsuki accidentally falls onto the train platform. When he wakes up, he finds himself faced with none other than... Kotoha!? To his surprise, Mitsuki has been reincarnated as the player character of "that" game...! With his vast knowledge and love of games, Mitsuki is determined to change fate this time. In order to protect Kotoha and the others from Ryou, Mitsuki must achieve a perfect happy ending! (Source: viviON, translated)