When a girl is having sex with more than one male at the same time and the first guy has blown his load inside her, and now it is your turn. As you go at it, you will 'stir the porridge' and be mixing the first guys cum with yours or however many have been before you.
The frothy mixture of both male and female ejaculate fluids that appears on your bed post coitus. This mixture generally leads to an argument over which sexual partner sleeps on which side of the bed. In turn this may lead to anger sex creating enough sex porridge to open up a soup kitchen
Man: "Wow that was incredible sex... I think I want the wall side of the bed."
Woman: "No I'm sleeping on the wall side... I have enough sex porridge on this side to offer customers the soup of the day."