Mostly ordinary Asuna had a unrequited crush on her classmate, Sakurako. If there was one unusual thing about her, it was that she had a penis.
Asuna was a "futanari". It was a secret she could tell no one.

She would raid her classmate's personal things. Usually she'd jack off into Sakurako's street clothes after school...... One day, Sakurako saw her.

Asuna panicked so hard that her hard-on went soft. But Sakurako's unexpected reaction changed everything.
"If you masturbate to me, I want to watch," she said.
Those words turned Asuna's flagging erection into a blasting off rocket.

From that day forward, curiosity fueled a relationship of masturbation---!

[From DLsite English]