18+ Majime na Kimi ni Hamaru Karada

Azusa Shirakawa was the star of her judo club all through high school, but now that she's in college, she's decided to move on and look for something new. Her friend Sae has some ideas for activities Azusa might enjoy, and sets her up on a hookup with their mutual friend Takuma—after all, Azusa and Takuma get along well, and they're both serious, straitlaced types who could use a chance to relax and unwind a little. The two of them are nervous at first, but Azusa quickly learns that underneath Takuma's nerdy exterior, he's got an unstoppable cock and boundless stamina. Azusa's used to being the toughest one around, but as she finds herself getting pounded into the mattress and drowning in pleasure, she discovers just how good it feels to lose.

(Source: Irodori Comics)