Sennou Kanryou. ~Zetsubou o Koete~

Main character's acquaintance Asami disappears. When she shows up in some few days, she is a flesh slave with no own will. She only sees manhood in men and demands to do crazy things to her. Protagonist decides to return her back to normal. He, Asami and her younger sister start to live together on a long and painful way to rehabilitation.

[From vndbreview]