I'm not reading 50 Shades of Gray as fanfic research. Besides, it seems that the BDSM practices described therein are all highly unsafe and not researched properly (dixit a BBC article about firemen complaining about the unsafe BDSM sex 50 Shades readers indulge in, resulting in said firemen needing to free the persons from brain-bleach-requiring scenes).
Tangentially related, one of the best "how to write" guides I've seen was posted to a.s.s sometime in the mid-90s, just as the web was starting to be a thing, and I've never been able to find it since -- it was styled as a guide for writing erotica so that it wasn't just "tab A in slot B", but that was just a specific application of the generally applicable advice.
If Google Groups wasn't stuffed with porn spam that somehow got retroactively posted everywhere, I'd try finding it...