Seems I Can't Help But Love You.

Hanatani Mitsuba is a simple university student who has had a crush on Shinomori Makoto for many years, but his confession is rejected.

He is introduced by his senior to his brother, Rui Shinomori, who teaches him how to have sex with other men...

One day, while they were having sex, Rui was seen by a senior colleague!

Mitsuba pretends to be in a relationship with Rui, but as they continue their fake relationship, he finds out about Rui's love for him and gradually begins to feel attracted to him.

Official Translations:
English, S.Chinese, T.Chinese, Korean