Screw Yourself: A remarkable percentage of people who have access to forking technology try this at least once or twice, with or without gender-flip, just to decide if that qualifies as sex or masturbation.
(Hint: it’s sex. Narcissistic, quasi-incestuous, oddly symmetrical sex.)
This is a companion discussion topic for the original entry at https://eldraeverse.com/2016/04/23/trope-a-day-screw-yourself