2012-07-07
Explanation
The comic shows a cat saying "Mow?" while a woman lies in bed, followed by a close-up of her horrified face and then an extreme close-up of her eye with the cat reflected in it. The humor comes from the bar graph at the bottom, which compares "Times a significant other has seen you masturbate" (a tiny bar) versus "Times a pet has seen" (a very tall bar).
The joke plays on the universal but rarely discussed experience of pet owners: that pets are oblivious to social norms around privacy, and because they are always freely wandering the house, they inevitably witness things their owners would prefer no one see. The woman's look of horror at the cat's innocent "Mow?" captures the sudden self-conscious realization that her pet has been a silent, non-judgmental witness. The graph format, a recurring device in SMBC, gives the joke a pseudo-scientific authority that makes the awkward observation even funnier.