Creepy wanky watchers

your partner(s) are getting down to business, clothing coming off and sex fantasies all ready to happen.  Then, looming in from the shadows comes 'that guy'.  He's staring silently, looking like a supervillain's henchman, or smiling maniacally, all alone.  He's standing a little too close for comfort, cock in hand, stroking himself off to your frolics.