I've been getting into drawing NSFW art this past year a lot more (I used to be strictly pen and paper and only drawing SFW), but I decided it was time to jump out of my comfort zone and try it out, and I've been enjoying it immensely. I've mostly been using my actual RP's as inspiration for such pieces as they tend to contain a fair bit of... 'erotica'.
I agree with Doctor, you should 100% get a tablet for this kind of work-- not only so it's easier to hide from friends and family (assuming you don't want them to see-- I know I sure as hell wouldn't share my nsfw pieces with anyone I know in my direct family!)
But as for actual resources, it's kind of embarrasing, but you can use like... actual porn as a reference. I use a site called Pornpics for certain positions I want. I also find studying other artists works for expressions or certain angles you want to portray very beneficial, we have to learn from the best after all. I use Gelbooru, Pixiv or even R34 search bars for this.
While Pinterest doesn't allow explicit NSFW content, there's PLENTY of reference there for things that can easily be turned NSFW. It's also incredibly useful when you can't draw a hand in a certain position or you want reference on how eyes are drawn from side-view.
So all in all, that's my process for finding NSFW reference, I'm not a master at it yet either but I've come a long way using these resources the past year.
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