From Vices and Devices:
Yumeji has a big problem, and no, it's not that he's gay (for now, anyway). Despite his good looks, excellent grades, and talent for sports, he's not getting any, and it has everything to do with the fact that he's impotent thanks to a traumatizing event back in middle school. Nothing seems to help, and since Viagra is apparently not a viable solution in Japan, he's practically lost all hope--until he meets underclassmen Seiji, who's violent, tactless, and says whatever is on his mind. Conveniently enough, he's also in love with Yumeji.

Chapter 3 oneshot Koibito Kijunchi
Mu's in love with his best friend Maru, but Maru doesn't take him seriously because, well, Mu's a huge slut who will sleep with anything that moves. So Maru suggests an "endurance contest": he'll sleep with Mu if Mu can hold out for ten days without sex.