It's forbidden love.
In the vein of the Beauty and the Beast, Saya no Uta or Kemonozume, those short stories tell an encounter between a human and an other-wordly beast.
Scary in appearance as they are beastly, you shouldn't be charmed by them if you were a normal person, nor have sexual relationship with them. Still, the author manages to make them likeable and cute, and sometimes intriguing, making us readers (and the MC) overflow with lecherous passion.
It's too bad the development is non-existent and things go quite fast to provide sex scenes, because I find those kind of taboo love stories with an unaccepted-by-norms being
fascinating. Also, their species customs and way of life until their encounter, the man getting used to living with the monster, and maybe turning his back to his own people etc, could be interesting things to develop.
But well, it's a hentai, you wouldn't really expect a developed story. I'm still bringing that up if any aspiring artist is up to the task.