It's rude to jack off in other people's beds.
John is house-sitting for his friend for the night and is feeling really tense.
John needs to relax.
John decides to lay down and read a good book.
John decides to watch some porn and whack off in his friend's bed.
John's friend is pleased John took such great care of the house while he was gone.
John's friend discovers some weird stains on the sheets and is grossed out.