As Kotaku previously mentioned, one of the manga artists who is responsible for that is Toshio Maeda—aka the self-proclaimed "tentacle master". In a recent interview, ANN's Rebecca Silverman asked Maeda about the appearance of tentacles in his adult manga.
"For some reason there were some issues about censorship about the sex scenes," Maeda told ANN. "When I was drawing sex scenes, sensual scenes in bed, editors always asked me not to be so extreme."
The reason why the editors were so fussy, Maeda said, was because it the manga was deemed obscene, the editors were the ones who would get arrested.
"So actually we mangaka didn't give a shit about that, but the editors," Maeda continued, "they really cared about that, so they always said, 'Please, not extreme,' But when we toned ourselves down, they said, 'Come on, you should do more.' So that's why I did the tentacle thing."
The rationale, Maeda continued, was that tentacles were not genitals, adding, "...they are like hands and legs or like uh, just body parts, so it's ok to go there." And since they're not private parts, then they're not necessarily part of a sex scene—and thus, not obscene.
Even Maeda admits that this was a "flimsy excuse", but it's an excuse that, for better or worse, has created an iconic trope for Japanese manga, anime, and even video games. It also made a tentacle master out of Maeda.
Interview: Toshio Maeda [ANN Thanks, Michael!]