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Ecchi and hentai are perfect, unlike real women. An image can possess the exact mood or attitude you want (joy, happy, fear, pain, intrigue), the women can have no physical flaws (flabby skin, unattractive face/hair/legs/waist/ass). The chick can actually be interested or excited to have sex, and express more emotion than "fuck me harder hurr durr." Since they are an idea, they are whatever you want them to be.
"Real" pornography, feels so dirty. Some prostitute who fakes orgasms or pleasure for the camera, when you know she is just getting $3,000 to fuel her drug habit. Not to mention her being treated like a piece of meat with holes in it by the male actors (I like happy sex pictures, consensuar, no fetishes or anything). Then there are the flaws, the unattractive moans or stupid hairstyles (pigtails on a 25 year old blegh, that's for little kids), and some fat balding guy's balls bouncing in front of the camera.
On top of that, hentai doujins can have an actual plot, with characters that have personalities, extremely well done art, be funny, ironic, cruel, or simply fun.
Fictional pornography - ecchi and hentai, have far more options, with far more quality, than "normal porn".