Zetsurin!

For as long as he could remember, young Keisuke has been troubled by a strange phenomenon—he always seems to accidentally fall over on top of girls at the worst possible time, landing hands-first on their chest or face-first in their crotch. One day, a magic angel name Ririi and her fairy friend Chappi descend from the heavens before Keisuke and reveal to him that he is in fact a legendary hero and these "accidents" were actually brought about as a result of a power that lay dormant in his groin. Having been awakened to his fate, Keisuke feels an energy surge within him and suddenly a large black gun barrel arises from his crotch—the holy genital gun, Zetsurin Buster. (Source: RocketNews24)