I think males are allowed, but it's a more controlled area - bigger police presence, less crowded, etc. I think the dense crowding is a big problem. These downtown squares like Köln's aren't that big, and during big celebrations, tens of thousands of people cram into them. If 20 Muslim dudes decided to surround and attack a woman, it would take far too long for the police to work their way through the crowd to help her. Basically it would be up to the German men immediately around her to help. They're not going to.