I don't think I could pin it down any more specifially than you did, but I definitely understand what you're talking about when you mention the difference between pin-up girls from the 40s and modern day pornographic pictures.
I think some of the difference may be that an erotic image (or other medium) is creating an appeal, something you see that makes you go "Oh, I *like* that." It gives a nudge towards arousal, and hints at something more carnal just behind the surface, but it's designed to be alluring as well as arousing. Sexual images (or other medium) seem to be much more focused on the physical aspect of arousal and acting on it. It's not so focused on being appealing, just depicting the sex.
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I think some of the difference may be that an erotic image (or other medium) is creating an appeal, something you see that makes you go "Oh, I *like* that." It gives a nudge towards arousal, and hints at something more carnal just behind the surface, but it's designed to be alluring as well as arousing.
Sexual images (or other medium) seem to be much more focused on the physical aspect of arousal and acting on it. It's not so focused on being appealing, just depicting the sex.