I write erotica, among other things. There are many resources you could use. Some books are especially for authors on how to write erotic scenes in fiction, but don't overlook the nonfiction sex books. Stargate just begs to have the boys find an alien Kama Sutra, so it helps to know the original.

https://www.goodreads.com/list/show/92004.Best_Books_on_Writing_and_Editing_Romance_Erotica_and_Love_Scenes

Kink books are very helpful for setting the scene and practical details.
https://www.goodreads.com/list/show/7908.Best_BDSM_non_fiction

General sex books are also useful:
https://www.goodreads.com/list/show/15121.Best_Nonfiction_Sex_Books_Ever_

And if you want to write polyamory, you need:
https://www.goodreads.com/book/show/54944.The_Ethical_Slut
https://www.goodreads.com/book/show/433498.Polyamory?from_search=true&from_srp=true&qid=5ZCkaB3iUM&rank=16

Here are some online resources:
http://booksbywomen.org/erotic-fiction-writing-tips/

https://spectrumboutique.com/journal/article/smut-with-style/

https://www.wattpad.com/633654066-how-to-write-fanfiction-the-do%27s-and-don%27ts-of

https://faithfulreader.livejournal.com/13008.html