Lordie, it's hard (no pun intended)....
Among the difficulties:
1. Describing body parts (porno-sounding vs. romance-novelly vs. medical vs. boring vs. slang)
2. Details of the act (just a little vs. nothing vs. implied vs graphic vs. flowery)
3. Emotions/internal thoughts (OMG vs. pure lust vs. none vs. mocking vs. analytical)
Just saying, it's not so easy.