/ic/ - Artwork/Critique

Why is it so hard to do imagination sex drawings? I think I've gotten to the point where I can decently copy figures and do figure drawing, but the second I try to add in a male actually doing the sex, the image turns flat and static because (I assume) I cant convey dynamic interaction between two characters and maybe proper form interaction as well, and maybe flat perspective
Pic related. Right is my attempt at imagination. Left is after I drew the pic on the right, went online and looked through a bunch of pictures till I found something similar to what I had in mind, edited it, and copied it quickly.
What exercises and practice should I do to remedy this? Or should I just always use a ref for this kind of stuff? (Was trying to draw shortstacks if anyones wondering) Also yes I am pretty beg