Netorareta!

Masazumi Tsurushita's father, Mineo, was an unemployed playboy, and Masazumi despised his father, calling him trash.
One day, Masazumi and Sayuki, his childhood friend whom he had always had a crush on, finally became lovers, and they began living happily and lovingly together.

However, soon after, Sayuki is deceived by Mineo and ends up having sex with him, mistaking him for Masazumi.

Even though she realizes it when it's inserted, she can't escape the pleasure of the thing, which is so much thicker than Masazumi's, and goes all the way.
After that, Mineo, a skilled technician, fucks Sayuki many times over, thoroughly teaching her the pleasures of sex.

Sayuki eventually falls head over heels in love with Mineo and his penis,
turns into a slutty woman who will do anything for him, and begins to distance herself from Masazumi...!

[Translated from Getchu]