the story is about a guy, his girlfriend, and his friend (main character).

the prologue was on the cafe. the guy said that he and his girlfriend never doing "it" yet. the girlfriend who doesn't know about sexual activity at all ask the main character to give her a "lesson". (the boyfriend don't know about it at all)

sexy time (not on the cafe)

the ending was also on the cafe with the guy, his girlfriend, and main character altogether. the boyfriend said that he and his girlfriend finally doing "it" but they cannot go all the way because he suddenly went limp. the girlfriend ask the main character to give her a "lesson" again. (also without the boyfriend know about it)

anyone know this H-manga title?

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found it!
"Muchi Muchi Miyako-san!" by Hige Namuchi

thx for your help

edit: oh.. it's me alone here 😐