Mamono Musume to no Seikatsu ~Alraune no Baai~

Reiku is a pharmacist, who everyday visits the grave of his dead lover Arun. One day, he notices a flower sprouting from her grave and begins to take care of the sprout. The sprout eventually transforms into a plant girl with the same face as his dead lover. The plant girl is a Alraune, a monster birthed from the blood of the dead Arun. Reiku treats the plant girl like a daughter and gives her a name Aruru. However, Aruru is a monster and needs to suck "vitality" of other creatures to survive, and the way that she gets to do it, is by eating Reiku's semen. Now, to feed his plant daughter, Reiku needs to offer his semen everyday to her, and he does it with pleasure, since he loves his plant daughter very much.

[Edited from description by fhc]