My dog has never been keen on men; not quite sure why, but she's nervous around strange men, cowering away a little bit, but never shown any signs of aggression, apart from barking like a loony at the front door!
Today, I was standing at the door of the shop where I work, just quickly popping my head round to check what time I was starting. Several people came in and out, dog was fine just standing there, as she always is. Suddenly, she went mental, barking and snarling at a bloke that tried to come in. She's seen him pretty much every morning on the school run and never paid any attention to him before, and it's so out of character for her to do this :( we've had issues with dog aggression (only when she's on a lead) but we've been training through this, and she's never, ever shown aggression to a person before. Fortunately she was on a short lead, and as soon as I told her to stop, she did, but it's thrown me a bit.