The setting? A black-haired boy named Harry and a blond-haired boy named Draco, who are enemies, are having sex beneath the Quidditch stands/in a dark recess of the Astronomy Tower somewhere/in a toilet cubicle at the Leaky Cauldron. Ho hum. What's the point? None? How did they get there? Ah, who cares, it's all about the porn. Let's slap a PWP label on it. There, problem solved, those who don't like won't read. Yay! But is there a difference between this scenario and, say, Jango Fett and Obi Wan Kenobi getting it on behind an obediently frozen-in-place division of clones (who are, of course, standing there to protect Jango and Obi Wan's great and t00by love)? If you take away the setting, is there a difference? Of course there is, the characters themselves are different. Right?
The characters themselves? Judging from my own observations, characterisation is probably the biggest talking point wherever you look. There are people who agree on a certain characterisation of, say, Draco and they happily read fics that contain "that" Draco and don't worry about all the other types of Dracos out there. Heck, there are people who are very adamant about their own interpretation of Draco, so much so that other interpretations cause five-page rants on the subject of whether Draco is abused or not. Why yes, I have got a mirror, thanks. ;)
Draco is nebulous in canon; for all we know, he's only like that with Harry and in reality, he's every older Slytherin's bitch. Even the fact that Slytherins are hierarchical is debatable -- it can be readily inferred from canon but it's not a fact. Maybe Draco is actually the fucktoy of every Slytherin in existence (ahaha Snape), and he compensates by being super mean to Harry and the Hufflepuffs (that sounds like a boy band). Or heck, maybe all Draco does in his spare time (besides coming up with ways to humiliate Potter) is bother his housemates to give money in order to save the starving children in Papua New Guinea. Whereas someone might not like my bigoted!Draco who isn't quite as suave as he'd like to be, I'm not a huge fan of sensitive!Draco with impeccable fashion sense. But I digress.
The point is, many HP characters are so crudely drawn in canon that they're up to all sorts of interpretations. Heck, we've even got feminine!Harry who cries at the drop of a hat out there, despite the fact that canon!Harry is about as likely to act that way as I'm about to decide that making McFlurries at McDonald's is the best career ever. So to say that porn written using certain characters is about those characters is to assume that the writer's interpretation of those characters is the One True Interpretation. Otherwise the story isn't about those characters at all, is it? At least not for those readers who do not agree with the author's interpretation of the character[s]. And if it's not about those characters, it could be about old Jango and Obi Wan if you change the names, appearances, and age them up a bit.
In the end, there is sex, and sex is good (when it doesn't involve Lucius/Mrs Norris...). I just wonder if people who say "I'm just here for the porn" actually mean that literally. Sometimes I also wonder if there shouldn't be a fandom for porn. :))