My theory is that they planned to clean the place. But they slipped so hard that they slipped out of their clothes and into each other. The impact also led to some old syringes falling from the bathroom to the side of the bed.
That or Patrick is an ass and I hope she kicks them both out in the cold in nothing but their birthdaysuits.
"I've actually just got this really crazy itch inside my butt, and we tried to administer drugs to make it stop, purely for medical purposes, and when that didn't help he offered to scratch it, but it was too far up for a finger so..."