Zetsurin!

From MangaHelpers: Amano Keisuke was born under a special star. Ever since he was young, he has encountered thousands of "lucky sukebe (ecchi)" situations that tend to have him end up being slapped by girls. One night, a falling star crashes into his house. The star turns out to be the magical angel Lili and her sidekick Chapi. They have come to save the world from the 100 demon lords that plot to destroy humankind by utilizing the secret weapon hidden inside Amano's... pants?! [DrCoke]