There's nothing inherently "pure and wholesome" about cartoons, whether still images on a page or in moving pictures; and the genre never has been "meant for children" exclusively or even mainly.

There have always been cartoons made for older audiences, some of them pornographic. Even today a number of popular cartoon shows made for TV such as South Park, The Simpsons, Family Guy, etc are made for an audience that includes adults as well as children, and whose content would go over the heads of little kids. Then there are movies like "Who Framed Roger Rabbit?" featuring the highly sexualized and large-breasted Jessica Rabbit, who was drawn, dressed and made to behave in ways that many women found disturbing and demeaning.

Though Mad Magazine was enjoyed by many older children, it was aimed just as much as teenagers and adults; and most - dare I say all - of its the content was beyond the grasp of very young children even if they could read.

Also, a lot of cartoon moving pictures made by the big name Hollywood studios in the 1920s-60s that were indeed meant mainly for children - including children who couldn't read comic books or strips on their own because they hadn't yet learnt to read - were explicitly racist, particularly against black people but also against East Asians and Mexicans. Nothing pure and wholesome about that.

Just as there have been erotic drawings and carvings for many thousands of years in a wide variety of cultures all around the world - as evidenced by ancient Greek pottery with explicit sexual imagery, the explicit carvings and artwork inspired of the Kama Sutra from 400-200 BCE, and the erotic statues adorning a number of Hindu temples from the Middle Ages, etc - the medium of cartooning has been used (mostly by men but I imagine some women too) to make and disseminate porn since the inception of cartoons, be they drawings or films.

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From its very beginnings, mainstream porn mags like Playboy and Penthouse used to publish quite a lot of porn in cartoon form.

While anime from or inspired by Japan is the most apparent and popular kind of cartoon porn around today, highly pornographic and scatological cartoons long were a popular genre in the US-based underground comix movement characterized by R Crumb in the 1960s. In the 60s, 70s and 80s (perhaps beyond), these sort of comics were widely read by tweens, teens and adults in the US. Whereas my impression is that while erotic manga is a tradition that in Japan goes way back, hentai as separate or specific sub-genre emerged since/during the 1990s, when the term originated.

Even some very popular mainstream cartoons made for popular audiences consisting mainly of children, or including children, were highly sexualized in their earliest forms. Early Minnie Mouse, for example.

Then there's Betty Boop, who was featured on kiddie TV programming all the time when I was a kid in the US in the early 1960s. Especially in her pre-code version, Betty Boop was a highly sexualized cartoon figure with an oversized babyish kewpie doll head and face, and a voluptuous grown woman's body. Betty was always dressed in scant, revealing clothes - basically just a short strapless slip with thigh-high stockings and garters and high heeled pumps - and had a personality that was childlike, coquettish, submissive and wanton. Betty Boop cartoons seemed designed to implant and inspire rape fantasies in the minds of those who watched - which unfortunately is the effect they had on me and many of my siblings and friends who watched them as small children.

Mainstream American cartoons like Dudley Do Right and Rocky and Bullwinkle featured a lot of female characters who were inappropriately depicted in highly sexualized ways - and they frequently featured disturbing story lines in which the females were seductive, submissive and used their "feminine wiles" on or against men in untrustworthy ways. In these cartoons, females were often menaced by dastardly villains and male violence. A famous theme/trope of Dudley Do Right was the "Damsel in Distress" in which a female character was subjected to kidnapping, bondage and nearly killed by an evil mustache-twirling older man, only to be rescued at the last minute by Dudley Do Right.