Least favorite plot device employed by way too many books you actually enjoyed otherwise
The Voodoo Punanny/Magical Vajayjay. Sex with the main character results in another character gaining magical abilities or being instantly healed of all wounds, both physical and mental.
Super Sex Organs aren't limited to paranormal books either. All that Bad Boy needs is some hot monkey sex with a Good Woman to cure him of all his emotional demons. The gender-swapped and equally overdone version of this trope is when the traumatized victim falls into the arms of a hero with "magical healing cock" and is cured, hallelujah! Because intimate psychotherapy is totes legit, amirite?
What overdone tropes are you sick of reading?