when you have sex with your ex-girlfriend/boyfriend & you don't even like them.

sex after the break up of a relationship that really doesn't mean anything

Is when your dick feels homesick and nostalgic.
Jokes aside, it's one of the easiest kinds of sex to get.
It may ruin your life an get you on the verge of "please come back to me" bullshit. However, it may also make you "whole" and keep you from idealizing the girl (and reflecting over the relationship you used to have)

n. The totally wrong but nearly universal phenomenon of fucking your Ex out of desperation, familiarity and/or pure poisonous hatred. (see: Hatefucking). Most common in the first dark six months after a breakup, and always trailed by feelings of self-loathing.

something you really want to avoid especially if you have moved on