Suezen! ~Tane o Nozomu Kanojo~

Protagonist was living alone until the day that a girl that he used to attend junior high together appeared at his door and said that she wants to have his baby. He fulfilled her request and had sex with her. What started as a surprising event become commonplace when one after another different girls kept coming to him with the same demand. He later found out that somehow those girls learned that his genes have the ability to conceive a child that would exactly be like their mom. After learning the truth he decided to offer all the help he can... [Translated from Getchu]