Its set in a world with demons and humans, the mc is a human and the ml is a demon. The demons look completely human, except they have horns on there head. The bigger the horn the more alpha the demon. When the ml gets aroused his horns grow out.
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Tabetemo Oishiku Arimasen
https://www.mangago.me/read-manga/tabetemo_oishiku_arimasen/
Am so happy i decided to read this masterpiece, I absolutely loved the secound couple! Its a bit bittersweet that he's not able to be a human and age with adora though. I loved the main couple also, am so happy they were able to have a happy ending.