The Case of the Inhuman Girlfriends: The Studious Succubus

The modern era, where humans and non-humans live side by side...
Kazuto Sakurai, a male student, had a girlfriend of a non-human (Myu) race.
Shinon Hamasaki is a succubus who is coming late to her sexual awakening.
They make for a happy couple.
But Kazuto, in the midst of adolescence, is not satisfied with just a platonic relationship,
longing for a step beyond a kiss, he spends his days in restless desire.
Then, one day...
Kazuto, overcoming the reserved Shinon, finally manages to have sex with her for the first time,
but right after, he faints due to extreme excitement and excessive ejaculation.
Undoubtedly, this was due to Shinon's nature as a succubus.

[From DLsite English]