One of my army of gorgeous female admirers asks in an e-mail:
"I see more manga porn than manga anything else. Could that possibly have something to do with the sales volume?"
Hmm...good question! I'm not especially knowledgable on manga, but I can imagine a number of possible reasons for this.
For one thing...to the general public, western comics are defined by their recognizable characters; manga is recognized by an artistic style. That gross oversimplification would cause a million comics and manga fans to puke their guts out, but I hope you know what I mean! When it comes to pornographic drawings, you can identify it as "manga" relatively simply by giving the girls big eyes and tiny noses and little pointy chins. By contrast, to define something as western comics porn, you'd have to have Wonder Woman or Supergirl or something.
That said, there IS a large "superheroine porn" subculture out there. I don't know how many people would recognize my current Tribe avatar as a character from a Disney Channel cartoon show called "Kim Possible" -- watch it sometime, it's awesome! -- but I recently discovered there are several webrings devoted to advocating/speculating on/depicting sexual relations between Kim and her adversary, the supervillainess Shego. I read some of the fanfic out of curiosity -- honest, officer, it was purely for research purposes -- and the funny thing was that it wasn't what I expected: crass male exploitation of wanting to see two hot chicks getting it on. These were mostly female fans, a la the Kirk/Spock subculture, and they really are FANS of the show; their erotica was created out of love for the characters. Perverse scary love, perhaps, but real affection nonetheless.
Which brings me to the next point: you see more manga porn than manga anything else because there's more porn anything than anything else. This is especially true for anything genre. All those authorized Star Trek series and movies and books and comics, numerous as they are, would be buried under the vast avalanche of ST porn. For all its bad reputation, there's something inherently cool about the idea of fan fiction to start with: instead of merely being passive consumers of corporate-produced entertainment, fans want to claim their favorites as their own to do with as they please.
That so much fan-created work is erotic and/or pornographic isn't an accident, and isn't solely because these fans are sexually frustrated; it's also that the culture is tangled in a lot of hypocrisy and mixed messages about sexuality, and the product put out by the mass media is simultaneously titillating AND denuded of overt real sex, owing to "standards of local decency" and the complaints of moral watchdog groups. We all know the Disney Channel is NOT going to show Kim Possible losing her virginity, or suggest that Shego might have Sapphic tendencies, so fan fiction depicting this automatically becomes more illicit -- and correspondingly more attractive -- but it's also a way of adding a kind of "realism" in some sense. The Star Trek series might tease their viewers with Seven of Nine or T'Pol in a tight leotard; the fans resolve the tension between the implied sexuality and lack of actual depicted sex by writing erotic stories. Or Photoshopping fake nude photos of Jeri Ryan and Jolene Blalock. (Look, the real world isn't a pretty place.) By contrast, I don't think anyone writes erotic fanfiction about porn stars. I could be wrong about that.
And finally: yes, there's a lot more sexually explicit manga to start with. Japan didn't have the artificially mandated definition of comics being for kids that we had imposed on us in America by Estes Kefauver (acting on the well-intentioned but seriously misguided guidance of Dr. Wertham) which led to the Comics Code and the eradication of comic books aimed at grownups for decades. For the Japanese, comics were a medium instead of a children's genre, so there was always adult stuff and porn widely available even as our stuff was being censored. Some of the stuff published in the States before the Comics Code came along was surprisingly risque -- and lurid, and violent, and gross -- and if American history had gone a slightly different way, "adult" comics might be as widespread here as Harlequin romances are.
Did I answer the question at all in the above rambling? I have no idea.