https://www.dlsite.com/books/work/=/product_id/BJ203349.html
https://coolmic.me/titles/2867 (English)
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"He'll Find Out if You Cum Inside Me...": Cheated on With a Massage

"Oh no...! If you come inside me... my family's gonna find out!!"
Ryoko's a married woman who started to work at a massage parlor in order to help out with her household finances. Though she thought it was a parlor with a special commendation, she was forced to wear an employee uniform that almost exposed her nipples.
Suddenly she had to have sex with her first customer...
While she was nervous as she gave a massage that she wasn't used to, her customer said he'd give her an example of what he wants, and Ryoko started getting massaged by her costumer instead. Ryoko naturally started panting as his fingers crawled across her body.
Though she thought, "Oh, no... This is gonna mean I'm cheating..." Her entire body started twitching.
He grabbed hold of her plump breasts and she started doing what he told her.
He stripped off her soaking wet panties and said, "I'm gonna loosen up the lymph inside you, too," and then shoved his manhood into a part of her only husband knew.
"Aaaaaaah! It's... so much bigger than my husband's!! I'm going crazy!"

Japanese RAW
Chapter 1: /g/1666219/5adcb6a389/

I like this title

Base +4, likeyouknowwhatever -6