Perhaps the reason is because this has a "caught in the act" feel to it. The photo suggests an erotic story.
One scenario that pops into my mind involves a married man in his late thirties who is visited by his now-grown college-age nephew. Both men have repressed gay emotions.
When the uncle accidentally walks in on his nephew whilst changing, desire and longing takes over. Soon they can't help themselves and are fucking madly on the couch.
And then the uncle's wife comes into the room. This is what she sees before screaming and passing out.
The source of this photograph remains a mystery, as does whether any more in a set exist. I ran it through Google's image search and nothing turns up.