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Himizu.
Pretty much every Inio Asano series has at least one. You can check out his latest ongoing series Mujina into the Deep.
U.P.O.
Sakuranbo Syndrome, alongside most Taku Kitazaki manga.
Shintaro Kago's ero-guro manga.
Ai Ren has this one particular intimate, very nice scene between two of the characters. Might be of interest if you're looking for variety in the way sex can be portrayed. Interesting setting/story, too.