Most erotic photography is focused on solo nudes, only a few photographers have a decent amount of "nude couples" in their portfolio. I can imagine it's harder to establish a tasteful, or sensual, atmosphere in this kind of photography.
But when it's done properly it provides a challenging and highly erotic viewing experience.
Something that's also important here is the distance photographers want to keep from pornography, and with a couple having sex, in front of the camera, things can get a bit blurry, I agree. So, is there a difference at all? Well yes and no, in my opinion pornography is heavily focused on the sexual act itself, while erotic photography should have a focus on atmosphere, sensuality, meaning and form. If you consider explicit sex in an image always porn, who am I to prove you wrong? I guess it's the same discussion we have about our site, and our games, in my opinion there is a difference, maybe best summarised in the words `intelligent´ and ´challenging´.
Anyway, I collected some great images ( from the Michelle7 website) that show quite explicit sex, photographed in a tasteful and artistic way.