Esprinter, your Bullmastiff sounds like a lovely dog, but I honestly think you're proving Gary's point. Gary's question was 'how does a dog demonstrate guarding ability without aggressive tendency?' By placing her body between you and a perceived threat your Bullmastiff is showing guarding instinct and aggressive tendency.

She is saying to the perceived threat, "Stop now or I will stop you." Now you know your dog a lot better than anyone here. It could be that if pressed your Bullmastiff might back down. That's very possible. But it doesn't mean that she isn't showing an aggressive tendency. How could a dog possibly guard if it isn't willing to show aggression?

Your definition as aggressive behavior being exhibited by 'junkyard dogs' is a false one. Though many people believe that an aggressive dog is an out-of-control terror, barking at fences and lunging towards people, that's just not true. That is the description of an untrained, unruly dangerous dog.

A truly aggressive dog is one that indicates it will use force to stop a threat. It's too bad that such a negative connotation is associated with the word aggressive. It's not necessarily a bad thing depending, of course, on the dog breed.