Folklore no Gochisou

When he bites me, my body gets horny... One day, Shinobu, a soft-hearted boy who loves children, takes Shion, a beautiful boy who was soaked in the rain in front of her house, into his care. At night, Shion suddenly licks the blood off of Shinobu's finger after he gets a cut. Soon after, his body starts becoming really sensitive and horny..! What's more, Shion transforms into an adult man right in front of her! Adult Shion then starts to teach the already turned on Shinobu how to do "all kinds of things". Soon after, Shion becomes fond of him, and begins to live in his house... (Source: Frontier Works)