Kanojo no Okaa-san ga Kanojo Yori Attouteki ni Ii Onna de Gaman Dekinai!!

The protagonist Yoshiki accepted a classmate's love confession not because she was his type,
but because he simply wanted a girlfriend.

Even when they have sex, he finds himself unable to get excited, and he's forced to
imagine his favorite porn situations in order to force himself to cum.

But everything changes one day, when he meets her mom...
His mind is fried by her hotness, and he gets madly horny...

That night, he arrived home, and furiously jerked off imagining the things he'd do to her.
But it wasn't enough.
She is all he can think about.
The more he sees her, the less he is able to hold back.
And soon enough, he lets loose...

[From DLsite English]