Manga Ore no Shiranai Murakami-kun.

From Fujoshi Bitches: After drinking at home with Murakami, his best friend from university, Minowa woke up to find Murakami riding him. Tempted by the erotic expression on Murakami’s face, Minowa unconsciously extended his hand… “You don’t have to do anything. Just be good and lie down.” Put in this appealing situation, his best friend who’s cuter than any woman, unexpectedly forbids touching! And above all, he’s the only one who knows the real Murakami who’s always the cool genius in University. Isn’t it impossible not to have any feelings for you after being in that sort of situation?! “Even I want to make you feel good!” However, there’s a secret as to why Murakami stubbornly rejects Minowa’s touch; one that he doesn’t want Minowa to find out no matter what.