In light of the misunderstanding that he and Michael had just the day before, the idea of Marietta assuming that they were together just wasn't as amusing for Anthony as it might have been any other time - well, as it might have been once he got past the intial what of it. And, of course, there was also the fact that he didn't appreciate any of the things she'd said about Michael because they frankly were not true. She was just saying things and Michael was going to listen because Michael was currently being very stupid when it came to his own self-worth. Whether she knew it or not, Marietta had just used that.
"What - no, we are not shagging." He didn't feel it was necessary to mention that Anthony was not in the habit of shagging anyone, ever. "We're mates, and mates tend to spend time together on a regular basis. And also? You don't know the first thing about Michael. He's not an idiot, he's not a burden, and he's not dragging anyone down - never has been and never will be." He gave Michael a brief, very pointed look before glaring at Marietta again. There was more he wanted to say, but he really didn't want to give Marietta any more ammunition if he could avoid it.