Kimajime Kyouei Joshi, Sei ni Oboreru

A girl named Yui Kawaga, who's devoted to her swimming club activities, had a dilemma. Her male senpai Okabe has done nothing but play around with girls, and she's trying to get him to be serious with his club activities.

"He's the lowest of the low, but the club needs his talent."

At the end of her inner struggle, she winds up offering him her body herself. Perhaps it's due to Okabe's skillful techniques, or maybe Kawaga's own natural sensitivity she didn't know she had, that from her first time having sex, she reaches powerful orgasms to such a degree that she can't even move. Ever since then, such transactions with him accumulate, again and again, and even her excuse, "this is all for the swimming club's sake," begins to fade while she's made to cum over and over—at last.

(Source: Original Creator, translated)