I hadn't thought about sex fics as a distinct genre, but the idea makes so much sense that I wonder why I didn't. (Probably overexposure to the idea that "fanfiction is porn" and "slash is porn," which I don't agree with but which is so prevalent in fandom that arguments against it simply waste time).

For my reading, I tend to prefer genslash, where the sex scenes are cornerstones of a larger plot and the characters don't spend all their time in bed, and I do think the standards for that type of fic are very different. I wonder why kinky fic gets a pass? Because there's not much of it outside fandom, maybe, so the readers want to encourage the writers to keep writing it?