This is a national regulation in Japan: male and female genitalia must be obscured with mosaic censorship, otherwise it constitutes a violation of the law. Failure to comply would result in Pixiv banning Atera sensei's account. Consequently, any work he uploads to Japanese websites must feature genitalia covered by mosaic censorship.
It is said this practice originated to protect the privacy of Japanese adult film actors. Notably, in Japan, the anus does not require mosaic censorship, as it is not considered a private area.