Mama no Oppai ~Ore no Doutei Milk ga Shiboritorareta Ken~

The protagonist, Makoto Mikimoto is a 3rd generation owner of a rural rental store.
However, he lost his mother early on in his life, and around the time he graduated from elementary school, he lost his father as well.
Thanks to that, Makoto now lives with and helps managing the adult-oriented store of his stepmother,Ayame.

Actually, there are two mother-figures in his life.

One is his stepmother: Ayame.
When Makoto was 12 years old, his father married her for his second marriage; and after his death from overwork, they still live together. She is a warm and kind, but sometimes strict, yet reliable mother.

And the other one. A lady who helped take care of something around the house until his father's second marriage.
She is his father's younger sister, making her Makoto's aunt, Kaho.

While helping these two beautiful older women, he never experienced loneliness and grown up quickly, but...
He failed his university exams, and is currently on his second tries.

And the cause of all of it--"boobs".

Ayame and Kaho, he can't get these two beautiful older women out of his mind, so much so he can't focus on his studies. Especially those beautiful breasts; if he reached for them, he could feel them anytime.

Worrying endlessly about that, one day he was helping around the store, when unexpectedly Kaho teased him about the size of his genitalia, making him lose self-confidence in his manliness.

Unable to concentrate on his studies, he worries about becoming impotent.
Seeing him like that, Ayame helps him, even while know what she is doing is immoral.

"It's okay, your stepmother will teach you every part of a 'woman', and make you a 'man'--"

[Translated from Getchu]