Ted felt insulted then. First because she was telling him he didn't understand when he did! Or, well, he thought he did. And then she took a jab at Muggles and Ted found himself frowning, fists clenched at his sides, feeling the full impact of her words but not saying a thing.
"Right" was all he said to her apology, as his arms folded over his chest. He did care about Narcissa, she was his best friend, but the fact of the matter was he loved Andromeda the way Narcissa wanted him to love her and that would never happen. Andromeda was the only person Ted would ever have eyes for and -- she would also never talk down about Muggles even if it was a slip up. Ted's parents were Muggles and every slight was like an insult against them.
Still, no matter how angry he was with her at the moment, he couldn't help but try once more. "I'm standing by you!" Ted said angrily, "And not because I have to because I want to!" because if that was the only reason Lucius stood by her side then what was the point?