It sounds from these various stories that not only is "screwing the pooch" making a bad mistake, but that you're so dumb you didn't even realize you were making a mistake while you're doing it (the guy who had sex with a dog thinking it was his wife.) Or the next morning, for that matter, until someone clues you in.

Or, it's got so many origin stories they're all wrong and we can go on speculating forever! Hurrah! Break a leg, everyone.