A guy finds out that he inhereted the incubus gene from his ancestor and needs to consume liqud. from a male, he reaches out to his childhood friend who lives next door to him and they have sex... After a while he finds out that his friend is becoming weaker, turns out that when he consumes his magic he is essentially also taking his life essence. He could however not take his life essence if both of them were in love with each other.
Chihira-kun wa Maryoku ga Tarinai