Dogs like people, have their own personalities. All boxers do not instinctively protect or if pressed, attack if their owners are threatened. I have seen many dogs bark and growl as if "protecting", only to retreat or run if the threat continues toward them. I have also seen many dogs, boxers included, that had to be trained to guard and if pressed to attack.
There are different levels of guard dog training. Some dogs can be trained to guard property and to bite trespassers. Dogs can be trained to guard by just standing their ground and warning the person by barking. They can also be trained to attack if owner or family is threatened. These dogs are trained to warn the threat by barking first and then biting if the someone presses the point (advancing toward owner or posing some immediate physical harm). I have witnessed many dogs trained to attack on command and to cease the attack on command. The difficulty in training a dog to guard and if need calls to attack is having a professional trainer train the dog and most importantly that the owner of the dog is responsible.