Sex Chuudoku ~Ano Ko ga Seido ni Naru made~ by Courreges Ace - Adult Sex Game

Otaku (protagonist) was a fat 3rd year college nerd.
One day on his way to school he injured himself, and was helped by a Ayano (heroine).
She was beautiful and pure, and like many, Otaku fell in love with her at first sight.

One day he saw her walking to school with another ***.
This was Teruo, a smart sensitive handsome nice guy.
Otaku tailed them to Ayano's house.

(Why is she taking him home? She loves ME.... ME...)
(... I won't forgive this....!!)

His heart shattered, Otaku began his secret revenge plan.