Hahaha so when I do use references im afraid its just digging round through the pornhub trenches, I'm honestly not phased by anything so I find that the easiest. Sorry though, probably not much help to you! I only reference maybe 20-30% of stuff, mostly more complex couple poses, or small elements like the angle of an ass or a mouth on a dick etc. Good news is life drawing and studying sfw poses will get you fairly far but there are things you can't easily learn without nsfw ref.
But yeah back to your question I keep a pureref file jampacked with screencaps, usually a few moments from each video i click on, and I'm never fussy really, if it's an interesting pose I'll save it. I have a very useful section of frame by frame hole pics haha ;;; I do also use clip studio 3D if I can't find what I want- Hearth tied up was using 3D ref, I couldn't figure out the crossing of limbs and I wanted something I could rotate!
However, I know there are diagrams and art posts about how to draw genitals which might be enough for you to learn, if anyone sees this and has any handy please link!