Pups are really easy to sex right from being born.

I bet the pups were all male because if there had been females, the difference would have been obvious to anyone. I'm surprised that the shelter worked didn't even know how to sex puppies. Maybe it is something they should be trained to do when working in such a place. After all, they have to rehome dogs and they can't do a good job if they don't even know whether a pup is a dog or a bitch.