Sex Chuudoku ~Ano Ko ga Seido ni Naru made~

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 boy.
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.

From DLsite English