Sei So Tsui Dan Sha

The tale of Ryu Ogawa, a timid young guy who gets bullied by three girls. When something extraordinary and supernatural occurs during a particularly violent harassment session by the girls on poor Ryu things turn for the kinky and interesting for the girls and Ryu. The girls use Ryu's equipment as a living sex toy, attaching it to anything and everything, including themselves, boning one another, friends, strangers and even Ryu himself! But the girls find it has a hidden cost...they soon find their own personalities are changing, bringing out their own deepest, darkest desires. (Source: AniList)