Kono S wo, Mi yo! - Cupid no Itazura

Synopsis

22-year-old Rin has two major problems; a complex about the tattoo-like birthmark on his butt and a crush on an older woman who thinks of him as her little brother. Rin hasn't allowed any woman to see his naked bottom since an incident when he was seven, which firmly entrenched his shame about his birthmark. When he finds his birthmark may grant him mystical powers over the opposite sex, will he finally be able to use his butt to his advantage? (Source: MangaHelpers)